distribution: Use After=network.target instead of multi-user

There is no need for osbuild-composer to wait until the whole multi-user
target is completed. It can be started earlier as it doesn't have any
dependencies in the target.

This can be a problem if there is a unit in the target that is not starting
and still is unrelated to osbuild-composer. There was a bug like this
with Plymouth where the service didn't finish and it was hanging. That
prevented osbuild-composer from starting and the user was left with
working SSH connection and shell, but composer-cli and systemctl start
osbuild-composer.service were both unresponsive.

Replace After=multi-user.target with After=network.target to start
osbuild-composer earlier.

distribution/osbuild-composer.service

@@ -1,6 +1,6 @@
 [Unit]
 Description=OSBuild Composer
-After=multi-user.target
+After=network.target
 
 # Weldr API needs a local worker by default.
 # Run `systemctl mask osbuild-worker@1.service`

distribution/osbuild-remote-worker@.service

@@ -1,6 +1,6 @@
 [Unit]
 Description=OSBuild Composer Remote Worker (%i)
-After=multi-user.target
+After=network.target
 
 [Service]
 Type=simple

distribution/osbuild-worker@.service

@@ -1,7 +1,7 @@
 [Unit]
 Description=OSBuild Composer Worker (%i)
 Requires=osbuild-local-worker.socket
-After=multi-user.target osbuild-local-worker.socket
+After=network.target osbuild-local-worker.socket
 
 [Service]
 Type=simple