Remove 'hangouts-chat' dependency

What does this MR do and why?

In this MR, we're removing the hangouts-chat gem, and its patches. We're using Gitlab::HTTP instead! 🚀

Screenshots or screen recordings

Before	Now

How to set up and validate locally

1. Setup a HangoutsChat integration (e.g. Settings/Integrations/GoogleChat)
2. Verify it works as expected
3. Checkout the bmarjanovic/hangouts-gem-removal branch
4. Verify that integration still works as expected 🎉

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.