Adapt REL_18_STABLE to its new status as a stable branch

Joe Conway <mail@joeconway.com>

Commit: 42625ecda28458e7446a83130135fdd061f21832
Author: Joe Conway <mail@joeconway.com>
Date: 2025-06-30T03:00:00Z
Releases: 18.0
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.

Files

PathChange+/−
Makefile modified +1 −1
README.md modified +2 −2
src/backend/nodes/gen_node_support.pl modified +2 −2