Enable "low risk" Rails 7.1 setting defaults

[mjankowski]

I want to start working through this file and either enabling or figuring out a migration path for the various settings here.

This PR is the result of doing a quick scan through the file -- I turned on anything which either I was pretty sure is not relevant to the app, reflects a practice we already had in place, or just otherwise seemed low-risk/sensible change.

I stayed away from anything which I either know needs a migration path or more contemplation first. Hoping to work through those in separate PRs.

Once they're all enabled and we're convinced all is well, we can delete the file and toggle application.rb to 7.1 defaults.

[mjankowski]

After the initial PR here I started making one-off branches for some additional setting changes, but realized that more of them were also pretty safe/boring, so pushed more commits here.

I (separately) also enabled the 7.1 defaults in application.rb, and the full spec suite passes with no warnings/deprecations/etc - so I think we're probably good from a narrow behavior perspective, and the only thing to sort out is migration paths.

I have left a few settings related to cache format changes, hash digest algorithms, marshalling formats, etc - unchanged for now - because I know there's separate discussion in misc other PRs about what changes to make there.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9322396) 84.78% compared to head (6059cf6) 84.77%.
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #28626      +/-   ##
==========================================
- Coverage   84.78%   84.77%   -0.01%     
==========================================
  Files        1039     1039              
  Lines       28163    28162       -1     
  Branches     4541     4541              
==========================================
- Hits        23877    23875       -2     
- Misses       3128     3129       +1     
  Partials     1158     1158              

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