Potentially update deprecation warning language

[GuySartorelli]

We've received feedback that the current language around deprecation warnings isn't as clear as we thought. The following feedback was received (numbered for easy reference):

1. The inclusion of deprecation warnings in CMS 5.4 changelog implies that action can be taken to avoid using that code immediately
2. Some may think that you absolutely must not use deprecated code - but in reality it being deprecated just means you should avoid using it, but can kick the can down the road until the major upgrade if you want.
3. The "will be" in a deprecation message isn't prominent enough - if looking at a block of deprecation warnings in logs or a changelog, it's easy to miss especially since it's in the middle of the warning developers see.

To resolve 1 and 2, a blurb at the top of the Deprecated API section could explain:

• the purpose of adding these to the changelog - that they're to give advanced warning that breaking API changes are coming
• that only some of the deprecation warnings can be avoided by taking action in CMS 5
• that it's best practice to avoid using deprecated code, but it's safe to use until its removal in a future major release

To resolve 3, we could add "in a future major release" at the end of the message. E.g:

- Config SilverStripe\Dev\DevelopmentAdmin.registered_controllers is deprecated. Will be replaced with "controllers" and "commands" configuration properties.
+ Config SilverStripe\Dev\DevelopmentAdmin.registered_controllers is deprecated. Will be replaced with "controllers" and "commands" configuration properties in a future major release.

Acceptance criteria

• Update the standardised deprecation message
• String replace all instance of this in CMS 5.4 branches
• Update 5.4.0 changelog to add a blurb, though do not string replace notices
• Post merge PRs, ensure all regular merge-ups are green, then merge-up all relevant branches and ensure updated deprecation notices are not accidentally merged in

PRs

• DOC Make it clear deprecations are removed in future major developer-docs#734
• FIX Update code to account for new "in a future major" language deprecation-checker#8
• ENH Make it clear deprecations are removed in future major silverstripe-subsites#642
• ENH Make it clear deprecations are removed in future major silverstripe-ldap#104
• ENH Make it clear deprecations are removed in future major silverstripe-admin#1927
• ENH Make it clear deprecations are removed in future major silverstripe-crontask#106
• ENH Make it clear deprecations are removed in future major silverstripe-linkfield#377
• ENH Make it clear deprecations are removed in future major silverstripe-externallinks#162
• ENH Make it clear deprecations are removed in future major silverstripe-assets#684
• ENH Make it clear deprecations are removed in future major silverstripe-campaign-admin#352
• ENH Make it clear deprecations are removed in future major silverstripe-realme#182
• ENH Make it clear deprecations are removed in future major silverstripe-staticpublishqueue#225
• ENH Make it clear deprecations are removed in future major silverstripe-siteconfig#194
• ENH Make it clear deprecations are removed in future major silverstripe-environmentcheck#130
• ENH Make it clear deprecations are removed in future major silverstripe-blog#815
• ENH Make it clear deprecations are removed in future major silverstripe-auditor#100
• ENH Make it clear deprecations are removed in future major silverstripe-graphql#636
• ENH Make it clear deprecations are removed in future major silverstripe-mfa#605
• ENH Make it clear deprecations are removed in future major silverstripe-userforms#1385
• ENH Make it clear deprecations are removed in future major silverstripe-versioned#503
• ENH Make it clear deprecations are removed in future major silverstripe-asset-admin#1561
• ENH Make it clear deprecations are removed in future major silverstripe-contentreview#278
• ENH Make it clear deprecations are removed in future major silverstripe-sharedraftcontent#283
• ENH Make it clear deprecations are removed in future major silverstripe-graphql-devtools#68
• ENH Make it clear deprecations are removed in future major silverstripe-framework#11712
• ENH Make it clear deprecations are removed in future major silverstripe-cms#3075
• ENH Make it clear deprecations are removed in future major silverstripe-elemental-fileblock#64
• ENH Make it clear deprecations are removed in future major silverstripe-elemental#1349
• ENH Make it clear deprecations are removed in future major silverstripe-advancedworkflow#581
• ENH Make it clear deprecations are removed in future major silverstripe-queuedjobs#466
• ENH Make it clear deprecations are removed in future major tractorcow-farm/silverstripe-fluent#958
• ENH Make it clear deprecations are removed in future major bringyourownideas/silverstripe-maintenance#257
• Merge branch '7.3' into 7 tractorcow-farm/silverstripe-fluent#959
• Merge branch '7' into 8.0 tractorcow-farm/silverstripe-fluent#960
• ENH Update notice with no replacement with clearer message. silverstripe-framework#11718

Important

After merging the above, reassign to guy to merge framework up to CMS 6

[lianna-blanca]

Adding "in a future major release." looks perfect to me - right where we'll be reading it anyway but clear enough at the end to not be missed :)