chore: update import paths and registry refs after move to gitea (#307)

Repo moved from GitLab to Gitea (git.unbound.se). Updates Go module path, all internal Go imports, Renovate registry pkg name, .buildtools.yaml registry block, and Dockerfile (drops obsolete GOPRIVATE + GITLAB_USER/GITLAB_TOKEN).

Reviewed-on: https://gitea.unbound.se/unboundsoftware/auth0mock/pulls/307

renovate.json

@@ -9,7 +9,7 @@
         "kubernetes"
       ],
       "matchPackageNames": [
-        "registry.gitlab.com/unboundsoftware/auth0mock"
+        "oci.unbound.se/unboundsoftware/auth0mock"
       ],
       "enabled": false
     }