Upgrade to rails 7 thejonroberts

[thejonroberts]

Resolves #734

Specs are all green via script/backend rspec.

New to the project, not sure how deployment works, but I assumed there are no rolling deploys involved. If there are seperate server instances, this commit will need to be removed, then re-added after deployment.

One annoying thing is deprecation warnings in specs (9 total):

DEPRECATION WARNING: `Rails.application.secrets` is deprecated in favor of 
`Rails.application.credentials` and will be removed in Rails 7.2. 
(called from remind at /app/app/mailers/checkin_reminder_mailer.rb:16)

I think it is out of scope here, and I don't know how prod env is handled -- but will need to migrate from secrets to credentials before upgrading to 7.2. (Warning gem could suppress warnings while running specs til then).

Other than than, I think this is ready.