We've long held the minimum at 3.80, but that's required more than
one workaround. Commit 0f39b70a6 broke it again, because it turns
out that exporting a target-specific variable didn't work in 3.80.
Considering that 3.81 is now old enough to get a driver's license,
and that the only remaining buildfarm member testing 3.80 (prairiedog)
is likely to be retired soon, let's just stop supporting 3.80.
Adjust docs and Makefile.global's minimum-version check to match.
There are a couple of comments in the Makefiles suggesting that
random things could be done differently after we desupport 3.80,
but I couldn't get excited about changing any of them right now.
Back-patch to v15, as 0f39b70a6 was.
Discussion: https://postgr.es/m/20220720172321.GL12702@telsasoft.com
3b474a2e66