units/getty@.service: use [Install]Alias= instead of static alias

units/getty@.service: use [Install]Alias= instead of static alias

In Fedora, kmsconvt@.service is starting to be used instead of getty@.service
to have nicer font handling. This means that we need the autovt@.service alias
point to the new unit. So far the alias was done through a static symlink
because there was little reason to change it. Let's use [Install] instead so
the decision which implementation to use can be made after installation.

https://bugzilla.redhat.com/show_bug.cgi?id=2429626
https://fedoraproject.org/wiki/Changes/UseKmsconVTConsole