Add jetty collection to gz-collections.yaml

[Crola1702]

As the title says

[j-rivero]

If I'm not wrong the appointed branches for Jetty are already setup in gazebo-tooling/gazebodistro#181 and should be the same here. So no main branches but probably starting with Ionic branches.

[Crola1702]

So no main branches but probably starting with Ionic branches.

Done. Can you check if everything is correct? @j-rivero

[j-rivero]

I think that major_version are not in sync with the package names. I.e: gz-cmake4 branch for gz-cmake5 package. So we need to update the major_versions too.