UX: Strip multiline comments in github oneboxes (#30040)

We were already stripping comments from GitHub issue/PR oneboxes, but the regex was not correctly matching multiline comments.

lib/onebox/engine/github_pull_request_onebox.rb

@@ -12,8 +12,6 @@ module Onebox
       include Onebox::Mixins::GithubBody
       include Onebox::Mixins::GithubAuthHeader
 
-      GITHUB_COMMENT_REGEX = /(<!--.*?-->\r\n)/
-
       matches_regexp(%r{^https?://(?:www\.)?(?:(?:\w)+\.)?(github)\.com(?:/)?(?:.)*/pull})
       always_https
 

lib/onebox/engine/github_repo_onebox.rb

@@ -11,7 +11,7 @@ module Onebox
       include JSON
       include Onebox::Mixins::GithubAuthHeader
 
-      GITHUB_COMMENT_REGEX = /(<!--.*?-->\r\n)/
+      GITHUB_COMMENT_REGEX = /(<!--.*?-->\r\n)/m
 
       matches_regexp(%r{^https?:\/\/(?:www\.)?(?!gist\.)[^\/]*github\.com\/[^\/]+\/[^\/]+\/?$})
       always_https

lib/onebox/mixins/github_body.rb

@@ -9,7 +9,7 @@ module Onebox
       end
 
       module InstanceMethods
-        GITHUB_COMMENT_REGEX = /<!--.*?-->/
+        GITHUB_COMMENT_REGEX = /<!--.*?-->/m
         MAX_BODY_LENGTH = 80
 
         def compute_body(body)