FlagFollowerUITest failing since new access policy was committed to core

Started working on this. So far, the view in flag_following seems to be quite dated, based on git blame.

What I did so far:

1. I think that @Berdir's hint about the cacheability is right. I changed FlagViewsRelationship to implement CacheableDependencyInterface, and implemented the necessary methods:
   1. If we're asked for a per-user relationship and the configured flag is not global, then the cache context will be ['user]
   2. Left the cache max age -1 (permanent)
   3. ...but in the cache tags, we return with the flag's cache ID (so if the flag will be changed somehow to be non-global or vice versa, then the view be recalculated).
2. Installed flag_following on a vanilla Drupal (10.3.x) and copied the re-exported YAML back to the module (without the config hash parts)
3. Then I cleaned up the changes in the file to help reviewing the changes... But maybe we don't want this cleanup?..
4. Fixed the two PHP CS nits in the plugin class.

I don't know whether it is needed to add some test coverage for the relationship plugin, let's see a green result for first (and ideally, some community review or hits from maintainers).

It is green, so try to get a review for now.

Thanks!