Adapt REL_19_STABLE to its new status as a stable branch

Joe Conway <mail@joeconway.com>

Commit: 3cd530e84b05080492dcf0f4c596ef9d58ced012
Author: Joe Conway <mail@joeconway.com>
Date: 2026-06-29T20:39:23Z
Adapt REL_19_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:
- 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