abi-compliance-check failure due to recent changes to pg_{clear,restore}_{attribute,relation}_stats()

My recent commit 688dc62, which was back-patched to v18, has made the
abi-compliance-check on buildfarm member baza unhappy:

    https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=baza&dt=2025-10-17%2013%3A11%3A11

Specifically, I replaced two functions related to lookups/privilege checks
for the new stats stuff in v18 with RangeVarGetRelidExtended().  FWIW I did
check codesearch.debian.net and GitHub for any third-party usage of these
functions before committing, and I found none.  Also, these functions are
only present in exactly one release (18.0).

My thinking was that this ABI breakage was probably fine, as I don't think
we really intended for these functions to be used elsewhere.  However,
since we have a buildfarm failure, I thought it best to broadcast my
thought process.  While I judged back-patching worth the risk, I could live
with reverting the change on v18 if anyone is concerned.

-- 
nathan