So that old installations that run setup-timezone without "-i" do not
need to execute manual steps to get sensible timezone configurations.
See https://gitlab.com/postmarketOS/pmaports/-/issues/2168 for more context
Fixes#2168
[ci:skip-build]: already built successfully in CI
Add post-upgrade file with a warning, so we don't symlink it to the
post-install script.
[ci:skip-vercheck]: no need to rebuild the package
Related: build.postmarketos.org#85