Remove the systemd-specific MOTD that says it is still in preview.
v25.06 is about to come out, the first stable release with systemd!
Any future adjustments to make it use fancy ascii colors etc. should be
made in postmarketos-base instead.
Part-of: https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/6669
[ci:skip-build]: already built successfully in CI
Oliver Smith
c8b5872b56