Fix CI for Rails 8.1

[aidanharan]

Get the tests to run against the Rails main branch.

Also, fix the following statement pool issue in the tests.

https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/actions/runs/12222831116/job/34093612408

/usr/local/bundle/bundler/gems/rails-410ad1c8a18e/activerecord/test/cases/connection_adapters/statement_pool_test.rb:8:in `<class:StatementPoolTest>': uninitialized constant ActiveRecord::ConnectionAdapters::StatementPoolTest::StatementPool (NameError)

      class TestPool < StatementPool
                       ^^^^^^^^^^^^^
	from /usr/local/bundle/bundler/gems/rails-410ad1c8a18e/activerecord/test/cases/connection_adapters/statement_pool_test.rb:7:in `<module:ConnectionAdapters>'
	from /usr/local/bundle/bundler/gems/rails-410ad1c8a18e/activerecord/test/cases/connection_adapters/statement_pool_test.rb:6:in `<module:ActiveRecord>'
	from /usr/local/bundle/bundler/gems/rails-410ad1c8a18e/activerecord/test/cases/connection_adapters/statement_pool_test.rb:5:in `<top (required)>'
	from /usr/local/lib/ruby/3.3.0/bundled_gems.rb:69:in `require'
	from /usr/local/lib/ruby/3.3.0/bundled_gems.rb:69:in `block (2 levels) in replace_require'
	from /usr/local/bundle/gems/rake-13.2.1/lib/rake/rake_test_loader.rb:21:in `block in <main>'
	from /usr/local/bundle/gems/rake-13.2.1/lib/rake/rake_test_loader.rb:6:in `select'
	from /usr/local/bundle/gems/rake-13.2.1/lib/rake/rake_test_loader.rb:6:in `<main>'