Maintenance: Improve custom gemfile loading to support multiple gem files.

Gemfile

@@ -233,4 +233,6 @@ end
 # ZAMMAD DEVS:  Consult the internal wiki
 #               (or else risk pushing unwanted changes to Gemfile.lock!)
 #               https://git.zammad.com/zammad/zammad/wikis/Tips#user-content-customizing-the-gemfile
-eval_gemfile 'Gemfile.local' if File.exist?('Gemfile.local')
+Dir['Gemfile.local*'].each do |file|
+  eval_gemfile file
+end