Restructure Flyte releases

[eapolinario]

Describe your changes

This PR restructures Flyte releases to happen only manually. We still produce a manifest, but the manifest is going to rely on the fact that at release time the artifacts (images, tags, etc) will be in place. This forces the user who's driving the release needs to be careful to use the same version across the two gh workflows:

1. generate_flyte_manifest
   i. in other words, generate_flyte_manifest now takes a next-version as input. This version is used to update the helm charts / kustomize.
2. create_release

Other changes:

• The checks gh workflow no longer produces tags + docker images on merges to master.
  • These new images are now only created at release time, when we kick off the create_release gh workflow.
• You'll notice that I didn't change the logic to retag docker images with the suffix -release (used in the prepare_artifacts make target).
  • The main reason behind that was to decrease the number of changes in this PR.
• I removed the bump-tags gh workflow completely and moved that functionality to create_release.

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

Screenshots

Note to reviewers