Remove pg_dump/pg_dumpall support for dumping from pre-v10 servers.

Remove pg_dump/pg_dumpall support for dumping from pre-v10 servers.

Per discussion, it seems like a good time to bump the minimum
supported version for various applications.  Our current policy is
to support at least 10 previous major versions, so this bumps the
minimum to v10 for the v20 release.  For reference, the minimum was
last bumped to v9.2 in 2021 for v15 (see commits 30e7c175b8,
e469f0aaf3, cf0cab868a, and 492046fa9e).  As in previous changes of
this sort, we aren't removing pg_restore's ability to read older
archive files, though it's fair to wonder how that might be tested
nowadays.

For ease of review, and to be able to put the indentation changes
in .git-blame-ignore-revs, I did not fix the indentation in this
patch.  I'll push a separate pgindent commit after these changes
are applied.

Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/adZ4j88Dq9r8y9_9%40nathan