Allow failure for Secret Detection job

This makes Secret Detection behavior consistent with the other Secure
analyzers

changelogs/unreleased/allow-failure-for-secret-detection.yml

+---
+title: Allow failure for Secret Detection job
+merge_request: 48152
+author:
+type: fixed

lib/gitlab/ci/templates/Security/Secret-Detection.gitlab-ci.yml

@@ -14,6 +14,7 @@ variables:
   stage: test
   image: "$SECURE_ANALYZERS_PREFIX/secrets:$SECRETS_ANALYZER_VERSION"
   services: []
+  allow_failure: true
   # `rules` must be overridden explicitly by each child job
   # see https://gitlab.com/gitlab-org/gitlab/-/issues/218444
   artifacts: