Part of fixing https://github.com/flutter/flutter/issues/139673, it will need to be cherry picked into the stable branch to fully resolve.

Originally I tried to come at this from `ci.yaml`, but the syntax does not exist to either conditionally include a dependency based on the branch we are on, or to disable a shard for a given branch (as opposed to enabling it which is supported). I could double every CI shard that uses Gold to try to serve my purpose, but it is already a very large and cumbersome file to keep up to date. Doubling it does not feel like the best solution. Using a RegEx is not my favorite, but I am using the same one we use in our CI, and left a note there to update it if it should ever change. Since there is already a whole infra built around it, I feel it is pretty safe so we can fix the stable tree.

We already had mitigated Gold affecting release branches in the past through flutter/cocoon (https://github.com/flutter/flutter/issues/58814), but https://github.com/flutter/flutter/issues/139673 exposed a rather rare edge case. A change was CP'd into the stable branch that introduced golden file image changes. Typically this would not cause an issue since any change that has landed on the master branch has golden files accounted for. In this case, the CP'd change on master has generated a different image on canvaskit due to another change that was not on stable. So when the CP landed, it generated a new image Gold had never seen before. Gold only tracks the master branch, so we cannot approve the image, and so cannot fix the stable tree.

This would disable the failing check on release branches and fix the tree.