Remove pg_upgrade support for upgrading from pre-v10 servers.

Remove pg_upgrade support for upgrading 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).

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