Releases: Fix name for [beta] releases

What

Fixes the name Beta version in its release page.

Why

Currently, a publication of a beta version of Octez will result in a release page named "Octez ."

How

Introduces a gitlab_release_beta_version variable in octez-release.sh to test if the release is a beta. If yes, the release page will be named correctly

Manually testing the MR

TO DO

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