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

contrib/intarray/bench/bench.pl

@@ -1,6 +1,7 @@
 #!/usr/bin/perl
 
 use strict;
+use warnings;
 
 # make sure we are in a sane environment.
 use DBI();

contrib/intarray/bench/create_test.pl

@@ -3,6 +3,8 @@
 # contrib/intarray/bench/create_test.pl
 
 use strict;
+use warnings;
+
 print <<EOT;
 create table message (
 	mid	int not null,