Fix: Remove the possibility to add multiple git-sync resources as DAG backends

Context

We have tried to implement multiple git-sync resources into one DAGS_CORE_FOLDER which was successful.

However when the feature was completed we noticed problems with this approach like the local dag path bleeds into DAG logic. After investigating we found reasons to rather not do it: https://github.com/stackabletech/airflow-operator/issues/721#issuecomment-4038282638

This now leads to the decision to not expose a possibility to add multiple dags in the Airflow CR.

What needs to happen

We want to remove the possibility to

  clusterConfig:
    credentialsSecret: test-airflow-credentials
    dagsGitSync:
      - repo: https://github.com/a-pretty-repo
        branch: feat/beautiful-branch
        gitFolder: "a-passably-folder"
      - repo: https://github.com/a-grave-repo
        credentialsSecret: git-credentials
        gitSyncConf:
          # supply some config to check that safe.directory is correctly set
          --git-config: http.sslVerify:false
        gitFolder: "a-well-formed-folder"

and rather have something like:

  clusterConfig:
    credentialsSecret: test-airflow-credentials
    dagsGitSync:
      repo: https://github.com/the-master-repo
        branch: feat/mighty-branch
        gitFolder: "all-my-magic"

as leaving this option open suggest a incomplete feature and might raise expectation on us to implement it.