Bump haml gem to 5.0.4

As pointed out in #49625, this version is needed to support Ruby 2.5.

See https://github.com/haml/haml/blob/master/CHANGELOG.md for more details.

Gemfile.lock

@@ -375,7 +375,8 @@ GEM
       google-protobuf (~> 3.1)
       googleapis-common-protos-types (~> 1.0.0)
       googleauth (>= 0.5.1, < 0.7)
-    haml (4.0.7)
+    haml (5.0.4)
+      temple (>= 0.8.0)
       tilt
     haml_lint (0.26.0)
       haml (>= 4.0, < 5.1)

changelogs/unreleased/sh-bump-haml-5-0-4.yml

+---
+title: Bump haml gem to 5.0.4
+merge_request: 20847
+author:
+type: performance