Update terraform template to avoid hardcoded master

changelogs/unreleased/terraform-ci-template-to-default-branch.yml

+---
+title: Merge branch 'mo-update-artifact-documentation' into 'master'
+merge_request: 61084
+author:
+type: other

lib/gitlab/ci/templates/Terraform.gitlab-ci.yml

@@ -56,6 +56,6 @@ apply:
     - terraform apply -input=false $PLAN
   dependencies:
     - plan
+  rules:
+    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
       when: manual
-  only:
-    - master