Add explicit activesupport version as CocoaPods dependency (#1590)

Summary:
## Changes in this pull request

All of the tests started failing today with a cryptic Ruby error. It turns out that a new version of `activesupport`, a dependency of CocoaPods was released and it introduced a bug. CocoaPods wasn't referencing a direct version of `activesupport` required, and so it was automatically trying to pull in this new invalid version.

This PR fixes the issue by explicitly specifying the last known good version of the dependency in the `Gemfile`.

### Checklist

- [x] All tests pass. Demo project builds and runs.
- [x] I added tests, an experiment, or detailed why my change isn't tested.
- [x] I added an entry to the `CHANGELOG.md` for any breaking changes, enhancements, or bug fixes.
- [x] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/main/.github/CONTRIBUTING.md)

Pull Request resolved: #1590

Test Plan: Tested and confirmed on my own fork of IGListKit this resolves the CocoaPods issue

Reviewed By: fabiomassimo

Differential Revision: D50013182

Pulled By: TimOliver

fbshipit-source-id: 623eab49dea5ff0b589b6f06ce589e99873dece4

Gemfile

@@ -1,5 +1,6 @@
 source 'https://rubygems.org'
 
+gem "activesupport", "= 7.0.8"
 gem 'cocoapods', '~> 1.12.0'
 gem 'danger', '~> 9.2.0'
 gem 'danger-swiftlint', '~> 0.32.0'