graph: Adjust defaults for user, group and SpaceProperties caches

The old default ttl of 30 minutes for the caches seems way too long. It
could cause outdated information users or groups to be returned for
quite a while. Especially since the TTL was reset every time an entry was
fetched from the cache. This is disabled now as well.

Fixes: #6320
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f