Update webmock gem to v3.9.1 for Ruby 2.7 deprecation fixes

See:
https://github.com/bblimke/webmock/blob/master/CHANGELOG.md

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/257438

Gemfile

@@ -413,7 +413,7 @@ group :test do
 
   gem 'shoulda-matchers', '~> 4.0.1', require: false
   gem 'email_spec', '~> 2.2.0'
-  gem 'webmock', '~> 3.5.1'
+  gem 'webmock', '~> 3.9.1'
   gem 'rails-controller-testing'
   gem 'concurrent-ruby', '~> 1.1'
   gem 'test-prof', '~> 0.12.0'

Gemfile.lock

@@ -547,7 +547,7 @@ GEM
       tilt
     hana (1.3.6)
     hangouts-chat (0.0.5)
-    hashdiff (0.3.8)
+    hashdiff (1.0.1)
     hashie (3.6.0)
     hashie-forbidden_attributes (0.1.1)
       hashie (>= 3.0)
@@ -1214,10 +1214,10 @@ GEM
     webfinger (1.1.0)
       activesupport
       httpclient (>= 2.4)
-    webmock (3.5.1)
+    webmock (3.9.1)
       addressable (>= 2.3.6)
       crack (>= 0.3.2)
-      hashdiff
+      hashdiff (>= 0.4.0, < 2.0.0)
     websocket-driver (0.7.1)
       websocket-extensions (>= 0.1.0)
     websocket-extensions (0.1.5)
@@ -1496,7 +1496,7 @@ DEPENDENCIES
   version_sorter (~> 2.2.4)
   vmstat (~> 2.3.0)
   webauthn (~> 2.3)
-  webmock (~> 3.5.1)
+  webmock (~> 3.9.1)
   wikicloth (= 0.8.1)
   yajl-ruby (~> 1.4.1)