Description of problem:
In RHEL 8 Red Hat moved the rpcgen binary out of the glibc-common package to its own package (rpcgen).  This had the side-effect of removing the rpcgen binary from the dovecot build environment.  Dovecot builds fine without it, but the missing dep causes rquota support to be disabled in the resulting dovecot build.

The solution is simple.  The dovecot.spec file simply needs to have rpcgen added as a builddep and rquota will start working again:

BuildRequires: rpcgen

Please note that while this bug is filed against RHEL 9 it is also present in RHEL 8 with the same solution.