[WIP] Update plugin generator to switch from rails secrets to credentials

[jrafanie]

Based on changes from:
ManageIQ/guides#553
ManageIQ/manageiq-providers-autosde#253

[jrafanie]

New plugins should not have existing secrets to worry about accidentally committing. Additionally, rails credentials aren't stored in the plugin directory, so we only need to .gitignore in the manageiq application.

[miq-bot]

This pull request has been automatically marked as stale because it has not been updated for at least 3 months.

If these changes are still valid, please remove the stale label, make any changes requested by reviewers (if any), and ensure that this issue is being looked at by the assigned/reviewer(s).

[miq-bot]

Checked commit jrafanie@07c0181 with ruby 3.1.5, rubocop 1.56.3, haml-lint 0.51.0, and yamllint
1 file checked, 0 offenses detected
Everything looks fine. 🍪

[miq-bot]

This pull request has been automatically marked as stale because it has not been updated for at least 3 months.

If these changes are still valid, please remove the stale label, make any changes requested by reviewers (if any), and ensure that this issue is being looked at by the assigned/reviewer(s).

[jrafanie]

Closing in favor of #23496 since Rails credentials is geared more towards teams sharing encrypted credentials, which is not what we need for VCR secrets.