Gradle Wrapper Bot should facilitate forward port issues

[jzheaux]

When the Gradle Wrapper Bot creates a PR to update Gradle, it does it for only one branch. When forward porting to other branches, no forward port issues attached to the other affected versions are created. Thus, those forward port issues must be created manually.

There are at least two alternatives to creating the forward port issues manually:

1. Configure the bot to create an upgrade PR for each branch. The downside here is that there is that it quadratically increases the amount of forward porting to be done
2. Configure the bot to include the PR number in the commit, like Closes PR-13456. In that case, the forward port bot will create the appropriate issues automatically while forward porting.

[marcusdacoregio]

For 2 to happen we would need to know the PR number at commit time, which is not possible, or we should create another workflow to set the commit message when the PR is opened. https://github.com/gradle/wrapper-upgrade-gradle-plugin does not have any option that help us doing that.

Another downside of 1 is that we have to, like Dependabot, keep the configuration updated with the supported branches. For dependabot.yml we now have an automation. I'm not sure about the effort to make one for the Gradle Wrapper update.

[jzheaux]

Perhaps a third would be a set of labels that could be used to trigger the creation of forward port issues.

Or, another might be perhaps there is a way to indicate a PR/Issue number to the Boot forward merge script so that tickets can be created and closed even when there is no "Closes" phrase in the commit message.

I haven't thought either of these through, just jotting them down before I forget.