Provide a policy around deprecations in KEDA

[tomkerkhove]

Provide a policy around deprecations in KEDA to help end-users be aware of what to do, by when, what the impact is and how to migrate.

Also, it should set expectations on when KEDA is allowed to remove things and break end-users.

Tasklist

• Define policy
• Add link to keda.sh / Project
• Add check in PR template for Core
• Add check in PR template for Helm

[tomkerkhove]

Braindump:

• Changes should be done in a backward-compatible fashion, deprecated and removed later on
• Once things are deprecated, they should be:
  • Announced on GitHub discussions (similar to `rolloutStrategy` is deprecated in favor of `rollout.strategy` in ScaledJobs keda#3552)
  • Have a warning in the KEDA logs
  • Have an entry in the deprecation section of our changelog
  • Have a callout in our GitHub release (similar to https://github.com/kedacore/keda/releases/tag/v2.8.0)
  • Have a representing issue that is tracked to remove it (labeled with breaking-change)