Use TS::Cache instead plain Hash in TimeZone.

Plain ruby Hashes are not thread safe.
Unify AS::TZ's lazy init maps.

There's no point in having 2 almost identical (@lazy_zones_map and @zones_map) lazy initialized TZ instance caches.
Remove AS::TZ.lookup(name).

The method doesn't really make much sense (find_tzinfo will succeed for any imput provided).
Update render with a spacer_template [ci skip]
Merge pull request #13119 from sbagdat/patch-1

Update render with a spacer_template [ci skip]
Removed redundant field name in the guide 

Fixes: #13108
[ci skip]
Merge pull request #13121 from pftg/patch-1

Removed redundant field name in the guide
Honor to RAILS_GROUPS env variable while loading gems with Bundler
config.assets.enabled isn't used anymore to disable Rails asset pipeline
Merge pull request #11796 from thedarkone/time-zone-thread-safety

AS::TimeZone's cache thread safety
Use genderless pronouns in API docs