pgsql: Doc: specify that the minimum supported version of Perl is 5.8.3

Doc: specify that the minimum supported version of Perl is 5.8.3.

Previously the docs just said "5.8 or later". Experimentation shows
that while you can build on Unix from a git checkout with 5.8.0,
compiling recent PL/Perl requires at least 5.8.1, and you won't be
able to run the TAP tests with less than 5.8.3 because that's when
they added "prove". (I do not have any information on just what the
MSVC build scripts require.)

Since all these versions are quite ancient, let's not split hairs
in the docs, but just say that 5.8.3 is the minimum requirement.

Discussion: https://postgr.es/m/[email protected]

Branch
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REL9_4_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/23c20e83d681ec74f2b9f51aa0bc19362a506fc7

Modified Files
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doc/src/sgml/install-windows.sgml | 3 ++-
doc/src/sgml/installation.sgml | 3 ++-
2 files changed, 4 insertions(+), 2 deletions(-)