Fix CI - Update workflow, setup, specs, and more

[sej3506]

Context:

In #995, a PR to add support for Rails 7.1, CI was failing.
The thing causing this first failure was issue number 1
below. Each additional fix here was yet another thing
CI failed on.

Things in this commit:

1. Split dummy:db:reset to dummy:db:drop and dummy:set:up
2. Replace outdated use of "MiniTest" with "Minitest"
3. Update session_spec.rb .serialize_cookies to handle old and
   new versions of Rack's handling of headers
4. Remove deprecated active record handling in application.rb
5. Update request_with_remember_token.rb remember_token_cookies
   handling of headers.

---

1. Setup & Github Workflow - Split dummy:db:reset to
   dummy:db:drop and dummy:set:up

This issue was discovered in #995.

I'm still looking into why exactly this is happening, but solution has
been consistent for me.

To reproduce the issue for yourself:
When CI runs for each version, it does so using
the specific Gemfile for the version.
Running
BUNDLE_GEMFILE=gemfiles/rails_7.1.gemfile RAILS_ENV=test bundle exec rake dummy:db:reset
and then
bundle exec appraisal rake
the Users table will not be found, because it is not created from this
command.

This issue only exists with the 7.1 gemfile that I've found.

Splitting the command dummy:db:reset to its component parts of dummy:db:drop and
dummy:db:setup on separate lines works - for all version. Using them on a
single line does not work, though.

2. Replace outdated use of "MiniTest" with "Minitest"

Use of "MiniTest" (vs modern "Minitest") was removed in version 5.19.0.
CI runs using version 5.20 of Minitest and was failing.

https://my.diffend.io/gems/minitest/5.18.1/5.19.0

3. Update session_spec.rb .serialize_cookies to handle old and
   new versions of Rack's handling of headers

The way Rack::Utils.set_cookie_header! works in rack 3.0.8
is different than previous versions.
Rack downcases keys at the class level now. This update supports both
versions of the method.

rack/rack@95d2f64

Also updating spec/support/cookies.rb and
spec/support/request_with_remember_token.rb for this.

4. Remove deprecated active record handling in application.rb

Rails 6.0 no longer uses config.activerecord.sqlite3.represent_boolean_as_integer
as it is always true now.
rails/rails@f59b081

As well, config.active_record.legacy_connection_handling has been
deprecated since Rails 7.0 and throws errors if used.

rails/rails#45835

5. Update request_with_remember_token.rb remember_token_cookies
   handling of headers.

The value might be a string or an array.

[sej3506]

I'll be squashing and merging, using the PR description as the final commit message.