xdg-autostart: Lower most info messages to debug level

It is expected for numerous autostart files to not be convertible to
corresponding units. The information is only useful for someone
debugging why a file might not be started, but it is not generally
useful for users in most situations.

As such, lower the warnings. Anyone wondering why an application is not
started will easily notice that the unit is not generated. From there it
will be somewhat harder to figure out why, but the overall trade-off is
still improved.

Fixes: #17305
(cherry picked from commit 47c1db67)