Raise requirement to Visual Studio 2022

Raise requirement to Visual Studio 2022

The use of _Generic from commit "Replace __builtin_types_compatible_p
with _Generic" causes compiler errors from VS 2019.  Apparently, that
compiler is just broken for that (even though it appears to support
_Generic in general, for example in commit 59c2f03d1ec).

Per discussion, just drop support for VS 2019 and require at least VS
2022.  This just updates the documentation about that.  In passing,
some information about VS 2026 is added.

Reviewed-by: Jelte Fennema-Nio <postgres@jeltef.nl>
Discussion: https://www.postgresql.org/message-id/flat/CA+hUKGL7trhWiJ4qxpksBztMMTWDyPnP1QN+Lq341V7QL775DA@mail.gmail.com