Fix CI

[aidanharan]

Version of 1.6.0 of the "pg" adapter is causing the CI to fail, shown below. Use v1.5.9 for the time being to get around the issue as we are not testing PostgreSQL anyway.

/usr/local/lib/ruby/3.4.0/bundled_gems.rb:82:in 'Kernel.require': cannot load such file -- pg_ext (LoadError)
Did you mean?  3.4/pg_ext
	from /usr/local/lib/ruby/3.4.0/bundled_gems.rb:82:in 'block (2 levels) in Kernel#replace_require'
	from /usr/local/bundle/gems/pg-1.6.0-x86_64-linux/lib/pg.rb:53:in 'block in <module:PG>'
	from /usr/local/bundle/gems/pg-1.6.0-x86_64-linux/lib/pg.rb:40:in 'block in <module:PG>'
	from /usr/local/bundle/gems/pg-1.6.0-x86_64-linux/lib/pg.rb:46:in '<module:PG>'
	from /usr/local/bundle/gems/pg-1.6.0-x86_64-linux/lib/pg.rb:6:in '<top (required)>'
	from <internal:/usr/local/lib/ruby/3.4.0/rubygems/core_ext/kernel_require.rb>:37:in 'Kernel#require'
	from <internal:/usr/local/lib/ruby/3.4.0/rubygems/core_ext/kernel_require.rb>:37:in 'Kernel#require'
	from /usr/local/lib/ruby/3.4.0/bundled_gems.rb:82:in 'block (2 levels) in Kernel.replace_require'
	from /usr/local/lib/ruby/3.4.0/bundler/runtime.rb:65:in 'block (2 levels) in Bundler::Runtime#require'
	from /usr/local/lib/ruby/3.4.0/bundler/runtime.rb:60:in 'Array#each'
	from /usr/local/lib/ruby/3.4.0/bundler/runtime.rb:60:in 'block in Bundler::Runtime#require'
	from /usr/local/lib/ruby/3.4.0/bundler/runtime.rb:52:in 'Array#each'
	from /usr/local/lib/ruby/3.4.0/bundler/runtime.rb:52:in 'Bundler::Runtime#require'
	from /usr/local/lib/ruby/3.4.0/bundler.rb:216:in 'Bundler.require'
	from /activerecord-sqlserver-adapter/test/cases/helper_sqlserver.rb:5:in '<top (required)>'