cleanup: remove unused GitHub Actions workflows and migrate PR Guard to Gitea

Deleted unused `.github` workflows and configurations, including obsolete CI/CD pipelines and PR templates, which were no longer executed after the migration to Gitea. Migrated the `PR Guard` workflow for hardcoded version checks to `.gitea/workflows/pr-guard.yaml`. Verified functional workflows in Gitea (e.g., Docker publish). Documented changes and rationale in session logs.

Signed-off-by: Stefan Mogeritsch <stefan.mo.co@gmail.com>

.gitea/workflows/pr-guard.yaml

@@ -0,0 +1,14 @@
+name: PR Guard
+on:
+  pull_request:
+    branches: [ "**" ]
+jobs:
+  no-hardcoded-versions:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Run hardcoded versions check
+        shell: bash
+        run: |
+          chmod +x config/quality/check-no-hardcoded-versions.sh
+          config/quality/check-no-hardcoded-versions.sh