In #19776 we discovered that basing both the base container *and* the
app container off of `.ruby-version` causes a chicken-and-egg problem
wherein upgrading Ruby versions requires a few stop-and-gos. Let's make
those stops reviewable as separate chunks instead: use a new
`.ruby-version-next` for building base images, not the `.ruby-version`,
thus allowing GitHub Actions to fire off an automated build (which, when
complete, we can take the SHA sum from and use it to open a second PR
updating the app image to use it, and `.ruby-version` can be updated to
match `.ruby-version-next`).
