Deprecate calling Rails.application.secrets

[p8]

Rails secrets have been deprecated in favor of credentials. Calling Rails.application.secrets should show a deprecation warning.

This requires #48470 which removes most internal calls to Rails.application.secrets.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[guilleiguaran]

I merged #48470, please rebase this

[p8]

Thanks @guilleiguaran !