Update tests and tested versions to cover Ruby 3.4 and Rails 8

[Cawllec]

Goal

With the releases of Ruby 3.4 and Rails 8 we need to ensure that the test setups run correctly on those versions. To that end, this PR:

• Bumps the leading test version of Ruby to 3.4
• Adds 3.4 to any multi-testing matrices
• Adds a Rails 8 test fixture, and tests it with Ruby 3.4
• Updates unit tests to allow changes in Ruby 3.4 to work correctly (as an example, {a: 1} as a string is '{a: 1}' in 3.4, where it was '{:a => 1}' previously)
• Adds any required dependencies to test fixtures for Ruby 3.4 (Such as to-be deprecated stdlib gems)

Changeset

The majority of the changeset is the rails8 app addition. I would suggest skimming everything in the features/fixtures/rails8/app directory as a lot of it will be generated from the rails application, and the rest will be identical to rails6/7 setups.

Notes

There are a few changes worth commenting on:

• We're currently skipping a JRuby unit test ensuring that java.lang.Throwables are processed correctly as this test fails when running on CI, but not locally. A ticket has been created to examine this further at a later date.
• We're currently only running the Rails 8 tests on Ruby 3.4, not earlier versions. The test fixtures are unable to start when running on CI, even though they run fine locally. A ticket will be created to examine this further as well.
• The test for SQL breadcrumbs with ActiveRecord bindings in Rails 8 has been skipped, as we've been unable to get the rails application to generate any bindings. A ticket for this has been created and we can revisit it at a later date. We're still testing to see if the SQL breadcrumbs without the bindings are created correctly.