Bump to 3.2 (#3214)

Three consumer-visible changes in this cycle that a build bump would under-signal:

- net6.0 dropped from TargetFrameworks (#3208). Anyone still targeting it does
  not get a worse version, they get no version.
- System.IO.Hashing's declared floor moved from 10.0.5 to 10.0.12 (#3210), on
  every target framework.
- ScriptEvaluateReadOnly now genuinely sends EVAL_RO where the server and command
  map allow it, so a script that *writes* on that API starts being rejected on
  7.0+ where plain EVAL used to let it through.

Plus the new surface from #3201 - RespResult, RedisKeyOrValue, the *Resp family,
and the un-gated RespReader API - which is additive but substantial.

assemblyVersion stays at 3.0 deliberately: it is decoupled from the package
version precisely so that a minor bump is not a binding break, and moving it
would be one for anyone with a compiled reference or a binding redirect.
Confirmed against a local pack - package 3.2.0, assembly still 3.0.0.0.

versionHeightOffset went with it: it was scoped to 3.1 via
versionHeightOffsetAppliesTo, so it stopped applying the moment the version
changed. Verified the computed version is identical with and without it.