Update if-elsif with case

https://github.com/rubocop-hq/ruby-style-guide#case-vs-if-else

app/models/project_services/jira_service.rb

@@ -137,10 +137,9 @@ class JiraService < IssueTrackerService
 
     return if issue.nil? || has_resolution?(issue) || !jira_issue_transition_id.present?
 
-    commit_id = if entity.is_a?(Commit)
-                  entity.id
-                elsif entity.is_a?(MergeRequest)
-                  entity.diff_head_sha
+    commit_id = case entity
+                when Commit then entity.id
+                when MergeRequest then entity.diff_head_sha
                 end
 
     commit_url = build_entity_url(:commit, commit_id)