Drop support for old dependencies/structures

[sergei-maertens]

The following patterns are outdated or obsolete and should no longer be supported:

(will require a bump to 2.0 to stick to semver)

• PyJWT 2.0+ required
• ZDS-client 2.0+ required (and ideally replaced with generic cg-api-client fork of gemma-zds-client)
• support DRF 3.12 and newer (now pinned to ~3.12)
• Drop vng_api_common.mocks, downstream needs to use requests-mock or similar instead. Lots of overlap with zgw-consumers, so possibly introduce that as dependency?
• Streamline setting names - now there's a prefix missing making it ambiguous
• Drop vng_api_common.compat
• Drop vng_api_common.notifications - make sure migrations have been executed to notifications-api-common
• Drop testing mixins - Open Zaak has its own implementation anyway
• Drop static assets/templates?