PG 14 pg_basebackup accepts --compress=server-zst option

https://www.postgresql.org/docs/14/app-pgbasebackup.html doesn't mention
"--compress=[{client|server}-]method". That first appears in the v15 docs.

And yet pg_basebackup doesn't complain about an invalid option.
(Technically, this is a bug; I first noticed it a week after copying a
script from a PG 15 server to five PG 14 servers, and running it quite a
few times without fail.)

$ pg_basebackup \
> --pgdata=$PGDATA \
> --dbname=service=basebackup \
> --verbose --progress \
> --checkpoint=fast \
> --write-recovery-conf \
> --wal-method=stream \
> --create-slot --slot=pgstandby1 \
> --compress=server-zst ; echo $?
pg_basebackup: initiating base backup, waiting for checkpoint to complete
pg_basebackup: checkpoint completed
pg_basebackup: write-ahead log start point: 256/BC000028 on timeline 1
pg_basebackup: starting background WAL receiver
pg_basebackup: created replication slot "pgstandby1"
42567083/42567083 kB (100%), 1/1 tablespace
pg_basebackup: write-ahead log end point: 256/BC000138
pg_basebackup: waiting for background process to finish streaming ...
pg_basebackup: syncing data to disk ...
pg_basebackup: renaming backup_manifest.tmp to backup_manifest
pg_basebackup: base backup completed
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