version: release/2026.1 should work fine. If you want to take advantage of the new overlapping support periods on versions, then this is the correct way to do it. (but of course, you must to remember to update manually before 2026.1 reaches its end-of-life)
version: esr should also work. The system is designed to support tags. It is implemented as git checkout $version.
You should not make this change in web.template.yml. You should make it in your site-specific containers/app.yml