Upgrade pact gem

What does this MR do and why?

This upgrade adds Rack 3 compatibility.

This gem is used for contract tests: https://docs.gitlab.com/ee/development/testing_guide/contract/

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

I tried running bin/rake contracts:merge_requests:pact:verify:get_diffs_batch locally but it fails for me even on master 🤷

Related to #396273