Automatically push release branch to Github

Context

Issue: #2442 (closed)

The main goal of the MR is to push a branch to git@github.com:tezos/opam-repository and to print out a URL to make it convenient for release managers to create PRs to git@github.com:ocaml/opam-repository (such at: https://github.com/ocaml/opam-repository/compare/master...tezos:opam-repository:foobar)

Manually testing the MR

I'm not really sure the best way to test this. There are some things I'm unable to test (for security reasons), so I'll need to leverage some help from a code owner. I believe @romain.nl will probably the person to help me test this MR.

What would need to test is the following:

• Create a tagged release (for testing purposes, I have been creating them in the nomadic namespace with the format v1224.x)
• Check the CI pipeline for the job named opam:release and ensure that it completes successfully.
• Double check for the presence of the release branch in tezos/opam-repository on Github.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR