Ensure the cherry pick operation only executes against trunk

[nigeljamesstevenson]

Submission Review Guidelines:

• I have followed the WooCommerce Contributing Guidelines and the WordPress Coding Standards.
• I have checked to ensure there aren't other open Pull Requests for the same update/change.
• I have reviewed my code for security best practices.
• Following the above guidelines will result in quick merges and clear and detailed feedback when appropriate.

Changes proposed in this Pull Request:

If a PR is merged with a milestone (or a milestone is added to a PR after it's merged) that targets a branch other than trunk, we shouldn't run the cherry-pick workflow.

Closes #38896

How to test the changes in this Pull Request:

Using the WooCommerce Testing Instructions Guide, include your detailed testing instructions:

1. Fork this repo and update the workflow file changed in this PR
2. Create a private slack channel for testing purposes. Call it whatever you want e.g. nigel-test-channel and obtain the Channel ID
3. Add the secrets required for this PR :

• secrets.CODE_FREEZE_BOT_TOKEN - use the value for secrets.TEST_ASSISTANCE_BOT_TOKEN instead (ask me if you don't know what it is) - i.e. you can use our test assistance bot for testing purposes
• secrets.WOO_RELEASE_SLACK_NOTIFICATION_CHANNEL - use the Channel ID from the channel created in step 2 for this (we do not want to send test messages to the actual WOO_RELEASE_SLACK_NOTIFICATION_CHANNEL)

4. Disable unnecessary workflows in your Fork to save bandwidth (you only need this workflow to be operational)

5. Create some milestones for testing purposes e.g. 9.1.0, 9.0.0, 8.9.0, 8.8.0

6. Create and merge a PR in your Fork targeting trunk for testing purposes. If you have one already, you can use it.

7. This PR will likely be assigned a valid Milestone e.g. 9.0.0 if the appropriate workflow is enabled on your fork.

8. Attempt to 'cherry pick' the PR created in step 6 into a release branch by assigning it another milestone e.g. 8.9.0

9. Validate that the cherry pick operation kicks off and attempts to perform the appropriate cherry-picking by progressing to the Prep inputs task
   

10. Create and merge a PR in your Fork targeting a release branch (such as release/8.9) for testing purposes. If you have one already, you can use it.

11. Attempt to 'cherry pick' this PR into a release branch by assigning it another milestone e.g. 8.9.0

12. Validate that the cherry pick operation does not kick off and does not attempt to perform the appropriate cherry-picking and does not progress to the Prep inputs task
    

Changelog entry

• Automatically create a changelog entry from the details below.

Significance

• Patch
• Minor
• Major

Type

• Fix - Fixes an existing bug
• Add - Adds functionality
• Update - Update existing functionality
• Dev - Development related task
• Tweak - A minor adjustment to the codebase
• Performance - Address performance issues
• Enhancement - Improvement to existing functionality

Message

Comment

[github-actions]

Hi @rodelgc, @Stojdza, @alvarothomas,

Apart from reviewing the code changes, please make sure to review the testing instructions as well.

You can follow this guide to find out what good testing instructions should look like:
https://github.com/woocommerce/woocommerce/wiki/Writing-high-quality-testing-instructions

[Stojdza]

It took me some time to set up everything and test it properly, but it all works as described 🚀