Adapt REL_18_STABLE to its new status as a stable branch

Per the checklist in RELEASE_CHANGES for the creation of a new stable
branch, this commit does the following things:
- Arm gen_node_support.pl's nodetag ABI stability, based on the contents
of nodetags.h.
- Update URLs of top-level README and Makefile to point to the new
stable version.

Makefile

@@ -20,7 +20,7 @@ all:
 all check install installdirs installcheck installcheck-parallel uninstall clean distclean maintainer-clean dist distcheck world check-world install-world installcheck-world:
 	@if [ ! -f GNUmakefile ] ; then \
 	   echo "You need to run the 'configure' program first. Please see"; \
-	   echo "<https://www.postgresql.org/docs/devel/installation.html>" ; \
+	   echo "<https://www.postgresql.org/docs/18/installation.html>" ; \
 	   false ; \
 	 fi
 	@IFS=':' ; \

README.md

@@ -12,9 +12,9 @@ and functions.  This distribution also contains C language bindings.
 Copyright and license information can be found in the file COPYRIGHT.
 
 General documentation about this version of PostgreSQL can be found at
-<https://www.postgresql.org/docs/devel/>.  In particular, information
+<https://www.postgresql.org/docs/18/>.  In particular, information
 about building PostgreSQL from the source code can be found at
-<https://www.postgresql.org/docs/devel/installation.html>.
+<https://www.postgresql.org/docs/18/installation.html>.
 
 The latest version of this software, and related software, may be
 obtained at <https://www.postgresql.org/download/>.  For more information

src/backend/nodes/gen_node_support.pl

@@ -107,8 +107,8 @@ my @nodetag_only_files = qw(
 # In HEAD, these variables should be left undef, since we don't promise
 # ABI stability during development.
 
-my $last_nodetag = undef;
-my $last_nodetag_no = undef;
+my $last_nodetag = 'WindowObjectData';
+my $last_nodetag_no = 479;
 
 # output file names
 my @output_files;