Lower minimum maintenance_work_mem to 64kB

Lower minimum maintenance_work_mem to 64kB

Since the introduction of TID store, vacuum uses far less memory in
the common case than in versions 16 and earlier. Invoking multiple
rounds of index vacuuming in turn requires a much larger table. It'd
be a good idea anyway to cover this case in regression testing, and a
lower limit is less painful for slow buildfarm animals. The reason to
do it now is to re-enable coverage of the bugfix in commit 83c39a1f7f.

For consistency, give autovacuum_work_mem the same treatment.

Suggested by Andres Freund
Tested by Melanie Plageman
Backpatch to v17, where TID store was introduced

Discussion: https://postgr.es/m/20240516205458.ohvlzis5b5tvejru@awork3.anarazel.de
Discussion: https://postgr.es/m/20240722164745.fvaoh6g6zprisqgp%40awork3.anarazel.de