Update release script to push gutenberg release branch

[mchowning]

Updating the release script to push the gutenberg release branch. I think we should do this because we update the gutenberg branch with the version number and the updated submodule hash is committed to gb-mobile release branch, but the referenced commit won't be pushed to the gb repository unless/until it is manually pushed. Until that is done no one else would be able to build the release branch.

To test:
Verify that the release script will push the gb branch.

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered if this change warrants user-facing release notes more info and have added them to RELEASE-NOTES.txt if necessary.

[mchowning]

👋 @SergioEstevao ! I'm inclined to update this PR to go further and just make a gutenberg PR as well. WDYT?

[SergioEstevao]

Good improvement.
As a follow up we should consider opening a PR too, we probably can add a custom template to there.