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/16894.1501392088@sss.pgh.pa.us

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d6f87a23d016d6cae9a062fb83d3d851ff1a5305

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(-)