Drop 7.2 support from docs and CI

[ndyakov]

go-redis is said to support the last three releases of redis. With 8.2 out and supported, 7.2 can be dropped from the documentation and the CI. Any tests for releases <7.4 can be removed.

If there are commands that were supported in 7.2 but dropped in 7.4, let's mark them for deprecation and deprecate them with the next redis release.

Although we are stating three releases, I don't see anything wrong with keeping the commands for a while longer if they don't interfere with the rest of the codebase.