Standardize order of use strict and use warnings in Perl code

The standard order in PostgreSQL and other code is use strict first,
but some code was uselessly inconsistent about this.

doc/src/sgml/generate-errcodes-table.pl

@@ -3,8 +3,8 @@
 # Generate the errcodes-table.sgml file from errcodes.txt
 # Copyright (c) 2000-2020, PostgreSQL Global Development Group
 
-use warnings;
 use strict;
+use warnings;
 
 print
   "<!-- autogenerated from src/backend/utils/errcodes.txt, do not edit -->\n";