Ship 3.2.0 API surface, and target the build at 3.2.0 (#3216)

Moves every PublicAPI.Unshipped.txt entry (StackExchange.Redis root and net6.0,
RESPite root) to its corresponding Shipped.txt, ahead of the 3.2.0 GA cut. Sets
versionHeightOffset -2, scoped to 3.2 via versionHeightOffsetAppliesTo, so that once
this merges the computed version lands on 3.2.0 rather than 3.2.2.