[ci.yaml] Update engine enabled branches

[CaseyHillers]

Remove branches that don't exist in the engine. This also adds the candidate branches to the enabled list.

This is to add an integration test to flutter/cocoon that validates the enabled branches match an existing branch.

flutter/flutter#84356

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See [testing the engine] for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the [CLA].
• All existing and new tests are passing.

[godofredoc]

How will this get translated to consoles and views?

[CaseyHillers]

It doesn't. In flutter/infra you have to match beta and stable to a specific version of this.

All this does is saves a step in the release process so you don't need to add the candidate branch each time.

[christopherfujino]

LGTM