bump minimum supported version of psql and pg_{dump,dumpall,upgrade} to v10

(This is for v20.)

I've brought this up a couple times in the past ~year, so here's a patch.
Supporting versions as old as v9.2 has become quite cumbersome, requiring
various version-specific branches and hacks.  I believe our current policy
is that we support at least 10 previous major versions [0].  For reference,
we last bumped the minimum to v9.2 in 2021 (commits 30e7c175b81,
e469f0aaf3, and cf0cab868a).

From "git show --stat":
    16 files changed, 569 insertions(+), 1836 deletions(-)

Thoughts?

[0] https://postgr.es/m/d0316012-ece7-7b7e-2d36-9c38cb77cb3b%40enterprisedb.com

-- 
nathan