Deprecate old URI syntax for caches

[FunkyBob]

Given it was removed from the docs 3 years ago, when multi-cache support was added:

​https://github.com/django/django/commit/673e6fc7fb243ed44841b9969d26a161c25733b3#diff-a7f182984504962fb780894ba7d4746e

it's about time to remove it.

This cleanup will also help the work to add a thread-safe, non-duplicating cache API.

PR - ​https://github.com/django/django/pull/1642

[Tim Graham]

It seems likely to me that people migrated to using the new syntax as part of the changes from the CACHE_BACKEND to CACHES setting and thus a justification for making this change without a deprecation. Objections?

[Aymeric Augustin]

Yes, since the old syntax hasn't been supported for years, and since it's very obvious and easy to fix for people still using it, we can remove it without going through a deprecation path.

I would mention it as a backward-incompatible change in the release notes. "The URL syntax still worked, although that wasn't documented nor officially supported. If you're still using it..."

[FunkyBob]

I've updated the docs.. please comment on working, etc.

[Tim Graham <timograham@…>]

In 4e9f800742c3048402acbaea67ec3c6bc3bd0935:

Fixed #21125 -- Removed support for cache URI syntax