Use perl warnings pragma consistently

We've had a mixture of the warnings pragma, the -w switch on the shebang
line, and no warnings at all. This patch removes the -w swicth and add
the warnings pragma to all perl sources missing it. It raises the
severity of the TestingAndDebugging::RequireUseWarnings  perlcritic
policy to level 5, so that we catch any future violations.

Discussion: https://postgr.es/m/20200412074245.GB623763@rfd.leadboat.com

src/backend/utils/Gen_fmgrtab.pl

@@ -1,4 +1,4 @@
-#! /usr/bin/perl -w
+#! /usr/bin/perl
 #-------------------------------------------------------------------------
 #
 # Gen_fmgrtab.pl