ci: use latest Python version supported by Red-DiscordBot, don't compile cog deps

[tigattack]

Initial Checklist

• Has @tigattack been added as a reviewer?
• If applicable, have the relevant project(s), milestone(s), and label(s) been applied?
• If applicable, have you added details of the cog to the readme as per README.md?

Details

Does this resolve an issue?
No

Changes

Features / Fixes

• Adds script to discover latest Python version supported by Red-DiscordBot
• Uses result to set up Python in GitHub Actions, ensuring the version used is always as up to date as possible whilst still being supported by Red-DiscordBot.
• Removes cog dependency compilation script/step.
  • This is a deliberate choice which increases the chance of breakage, but better enforces requirements-dev.txt being kept up to date.