Update documentation to reflect disabled GitLab mirror

Co-authored-by: pancetta <7158893+pancetta@users.noreply.github.com>

Author: Copilot

README.md

@@ -33,8 +33,7 @@ implemented.
     [PETSc](http://www.mcs.anl.gov/petsc/) (through
     [petsc4py](https://bitbucket.org/petsc/petsc4py))
 -   Continuous integration via [GitHub
-    Actions](https://github.com/Parallel-in-Time/pySDC/actions) and
-    [Gitlab CI](https://gitlab.hzdr.de/r.speck/pysdc/-/pipelines) (through the [GitHub2Gitlab Action](https://github.com/jakob-fritz/github2lab_action))
+    Actions](https://github.com/Parallel-in-Time/pySDC/actions)
 -   Fully compatible with Python 3.10 - 3.13, runs at least on Ubuntu
 
 ## Getting started

docs/contrib/02_continuous_integration.md

@@ -112,6 +112,8 @@ pytest -v pySDC/tests
 
 ## Running CI on HPC from pull requests
 
+> :warning: **Note:** The GitLab mirror integration is currently disabled due to technical issues. This section describes functionality that is temporarily unavailable.
+
 By syncing the GitHub repository to a certain Gitlab instance, CI-Jobs can be run on HPC machines. This can be helpful for benchmarks or when running on accelerators that are not available as GitHub runners.
 
 For security and accounting reasons, a few extra steps are needed in order to run the contents of a pull request on HPC: