Add FEniCSx adapter to the system tests

[MakisH]

Related to #448 and triggered by #714.

To add the new component:

• In tools/tests/:
  • Added FEniCSx in components.yaml.
  • Added component-templates/fenicsx-adapter.yaml.
  • Added a fenicsx_adapter Docker stage in dockerfiles/ubuntu_2404/Dockerfile (not adding new components to Ubuntu 22.04 anymore).
  • Added a FENICSX_ADAPTER_REF variable in reference_versions.yaml.
• In the GitHub Actions workflows (added after merging, in 87579cf):
  • Added the fenicsx-adapter to the gather-refs step to get the latest commit.

To add the tests:

• In tools/tests/tests.yaml added the missing test cases, referenced them in release, and created a fenicsx-adapter test suite.
• In the metadata.yaml of each FEniCSx-related tutorial, commented-out the respective entries for FEniCSx, as the component now exists.
• Generated the reference results for the fenicsx-adapter test suite.