Add new encrypted settings secret for rails

What does this MR do?

Add the secret generation for the new encrypted setting key being introduced in: gitlab-org/gitlab!43220 (merged)

• encrypted config files MR gitlab-org/gitlab!43220 (merged)
• ldap encrypted credentials MR gitlab-org/gitlab!45712 (merged)
• documentation MR gitlab-org/gitlab!48090 (merged)
• omnibus MR for generating the secret gitlab-org/omnibus-gitlab!4687 (merged)
• adding to usage data MR gitlab-org/gitlab!48210 (merged)

Related issues

Part of gitlab-org/gitlab#238483 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened