Use Rails.application.secret_key_base directly

[segiddins]

This helps avoid the rails 7.1 deprecation warning about using Rails.application.secrets when no secret key base is explicitly configured, when Rails.env.local? || ENV["SECRET_KEY_BASE_DUMMY"]

Description

Fixes # (issue)

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works

Screenshots & recording

Manual review steps

1. Step 1
2. Step 2

Manual reviewer: please leave a comment with output from the test if that's the case.

[adrianthedev]

It seems that Rails.application.secret_key_base does what we already did in the removed method.
I didn't know that.
Thanks @segiddins!

[jpawlyn]

Would it be ok to have a similar PR against main @adrianthedev to avoid the deprecation warning for Avo 3 ? If so happy to do it.