Configure gitlab-ci.yml and reactivate the pipelines

For more information, refer to Creating release branches for RHBQ documentation procedure.

1. In the scripts that are triggered by this pipeline, we need to set from where we are single-sourcing the updates and where the update needs to land. Set both values in the gitlab-ci.yml file to match a specific release and upstream branch, such as 3.8.
2. QUARKUS_BRANCH=3.8 -> source of truth (Upstream Quarkus branch)
3. TARGET_BRANCH=quarkus-3-" -> RHBQ repository branch for a specific release

An example with RHBQ 3.8:

In the release branch, open the gitlab-ci.yml file and change the value of TARGET_BRANCH="main" to the value of the new release branch. For example, if the release branch is `quarkus-3-8`, change the value to TARGET_BRANCH="quarkus-3-8". That way, when you run the pipeline, the resulting MR targets the correct release branch, not `main`.
Set QUARKUS_BRANCH the same way.

1. Reactivate any pipeline schedules you deactivated at the beginning of the documentation content freeze. Run the pipelines manually to verify that they work.