Upgrade Rugged to 0.28.3.1

This is a security release:
https://github.com/libgit2/libgit2/releases

Gemfile.lock

@@ -843,7 +843,7 @@ GEM
     rubyntlm (0.6.2)
     rubypants (0.2.0)
     rubyzip (1.2.2)
-    rugged (0.28.2)
+    rugged (0.28.3.1)
     safe_yaml (1.0.4)
     sanitize (4.6.6)
       crass (~> 1.0.2)

changelogs/unreleased/sh-update-rugged-0-28-3.yml

+---
+title: Upgrade Rugged to 0.28.3
+merge_request: 31794
+author:
+type: security