Set minimum required version of zstd as 1.4.0.

Set minimum required version of zstd as 1.4.0.

It emerges that our new zstd code depends on features that were not
present (or at least not enabled by default) in zstd before 1.4.0.
That's already four years old, so there's little reason to try to
make our code work with something older.  Instead make configure
check that zstd is at least 1.4.0, and document this requirement.

Justin Pryzby (doc changes by me)

Discussion: https://postgr.es/m/efbd94cd1102f99fd1300e44905ee4a994ee2ef6.camel@gunduz.org