Set cacheable metadata for the redirect

(I corrected the example given in the IS, since that is an example of cache context, not cache tag.)

For a list of all the available cache contexts (from Drupal core), see Cache contexts → . For cache tags, I did not find a list, but on #2145751-30: Introduce ENTITY_TYPE_list:BUNDLE cache tag and add it to single bundle listing → , I noticed there is a tag for configuration objects, which I guess is invalidated when that configuration object is changed.

Given than in 🐛 Use the maintenance_mode service Active the class dependencies will be changed, I am postponing this. The class will probably need just two or three cache tags.