units: rework growfs units to be just a regular unit that is instantiated

units: rework growfs units to be just a regular unit that is instantiated

The systemd-growfs@.service units are currently written in full for each
file system to grow. Which is kinda pointless given that (besides an
optional ordering dep) they contain always the same definition. Let's
fix that and add a static template for this logic, that the generator
simply instantiates (and adds an ordering dep for).

This mimics how systemd-fsck@.service is handled. Similar to the wait
that for root fs there's a special instance systemd-fsck-root.service
we also add a special instance systemd-growfs-root.service for the root
fs, since it has slightly different deps.

Fixes: #20788
See: #10014