Add api_versions to /api/v2/instance (#31354)

spec/requests/api/v2/instance_spec.rb

@@ -18,6 +18,7 @@ describe 'Instances' do
         expect(body_as_json)
           .to be_present
           .and include(title: 'Mastodon')
+          .and include_api_versions
           .and include_configuration_limits
       end
     end
@@ -32,6 +33,7 @@ describe 'Instances' do
         expect(body_as_json)
           .to be_present
           .and include(title: 'Mastodon')
+          .and include_api_versions
           .and include_configuration_limits
       end
     end
@@ -53,5 +55,13 @@ describe 'Instances' do
         )
       )
     end
+
+    def include_api_versions
+      include(
+        api_versions: include(
+          mastodon: anything
+        )
+      )
+    end
   end
 end