[1.2] Reinstate Rails integration test stubs for Rails 5.0 -> 7.2

[chadlwilson]

backport/cherry-pick of #313 and #322 to the stable/maintainbled 1.2.x branch

• Generated new Rails dummy apps from scratch for all versions we currently test
• Lifted all of the current initialization tests into the shared examples to run for all Rails
• Added back the chunked rack monkey-patch test that was there for the earlier rails 4.0 era stubs
• Added some gitignores to try and make it easier to generate and add new minimal stub Rails applications
• Added a test that will fail if we forget to add new stub tests that actually boot/initialize Rails for a new version

Looks like

  rails 5.2
    it should behave like a rails app
      runtime
        initializes (pooling by default)
        initializes threadsafe!
      initialized
        loaded rack ~> 2.2.0
        booted with a servlet logger
        sets up public_path
        disables rack's chunked support (by default)

Compared to #322 and 1.3.x/master

• the initialization/runtime tests are different (left unchanged here) since there have been changes made on master/1.3 to the defaults.
• the 1.2.x logger silence mechanism works fine on older Rails versions since it does not depend on ActiveSupport::LoggerSilence on 1.2.x so no need to change production code here
• omits the Rails 8.0 tests, since this is a 1.3.x feature
• Changes logger expected class since this has evolved on 1.3.x/master
• Re-cherry-picks a couple of older changes to re-apply some earlier-omitted differences.