Define a Redis minimum supported version policy

This issue has been migrated from [#14031](https://github.com/matrix-org/synapse/issues/14031).

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Like #12983, but for Redis.

https://endoflife.date/redis quotes their upstream support policy, with new major versions releasing roughly annually.

[Noticed here.](https://github.com/matrix-org/synapse/pull/13981#discussion_r984598540)

Rough todo list:

- Check we're okay with Redis's release cadence
- Consult packagers
- Mention Redis on https://matrix-org.github.io/synapse/latest/deprecation_policy.html
- Decide if we want to enforce this in Synapse
- Announce in release notes and upgrade notes.