Bump Rails cache key

This appears to be necessary when a new Ruby is recompiled with updated
system libraries.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/326348

.gitlab/ci/global.gitlab-ci.yml

@@ -27,7 +27,7 @@
 
 .rails-cache:
   cache:
-    key: "rails-v4"
+    key: "rails-v5"
     paths:
       - vendor/ruby/
       - vendor/gitaly-ruby/