pgsql: Increase minimum supported GNU make version to 3.81.

Increase minimum supported GNU make version to 3.81.

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

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3b474a2e660f53f0a5aebfbb4c706d30aa145c16

Modified Files
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doc/src/sgml/installation.sgml |  2 +-
src/Makefile.global.in         | 13 ++++++-------
2 files changed, 7 insertions(+), 8 deletions(-)