Removed information about releases < 1.7.
Merge pull request #1973 from haikoschol/master

Removed information about releases < 1.7.
Fixed #21445 -- Clean up misuse of null in quickElement.

Thanks to trac user parsch for the report.
Fixed #21012 -- Thread-local caches, like databases.
Updated docs to describe a simplified cache backend API.
Removed create_cache function.
Removed all uses of create_cache.

Refactored the cache tests significantly.

Made it safe to override the CACHES setting.
Looked up the template_fragments cache at runtime.