doc(plugin.xml): Update engine requirements recommendation for Cordova

[GitToTheHub]

• Make clear, that the <engines> tag in plugin.xml is still required
• Wording suggested by @erisu in chore!: bump 9.0.0-dev and cordova-ios to 7.0.0 cordova-plugin-camera#939 (comment)

Platforms affected

Motivation and Context

Description

Testing

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary

[GitToTheHub]

For me it's currently not clear, if cordovaDependencies in package.json is optional for the current cli. If it is optional, this PR should fit everything,

[erisu]

Yes, cordovaDependencies is optional. When not defined it should just install latest or whatever version they passed in.

[GitToTheHub]

Thanks for clarifying this :)

[GitToTheHub]

I tested cordova-plugin-camera wihout cordovaDependencies and it installed just fine