pgsql: Use a more portable way to get the version string in PostgresNod

Use a more portable way to get the version string in PostgresNode

Older versions of perl on Windows don't like the list form of pipe open,
and perlcritic doesn't like the string form of open, so we avoid both
with a simpler formulation using qx{}.

Per complaint from Amit Kapila.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8bdd6f563aa2456de602e78991e6a9f61b8ec86d

Modified Files
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src/test/perl/PostgresNode.pm | 6 ++----
1 file changed, 2 insertions(+), 4 deletions(-)