CI, Release: Rename octez release

What

Renames releases to Octez releases. It changes the release pages name along with the tag that triggers the octez release octez-vx.y or octez-x.y.

The tests are updated accordingly.

Why

Preliminary work to introduce releases for other tezos/tezos projects, such as etherlink.

How

• Renaming files
• Adding prefix octez- to matched tags
• Fix octez-version parsing

Manually testing the MR

• See the new test release https://gitlab.com/nomadic-labs/tezos/-/releases/octez-v5555.6
• Check that https://gitlab.com/nomadic-labs/tezos/-/tags/v5555.6 is indeed not an Octez release

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