[__debconf_set_selections] Fix --file not being supported

Even if deprecated, the parameter *must* be supported, which isn't the case
right now.

This was due to a misunderstanding of how deprecating parameters work, see:
https://www.cdi.st/manual/latest/cdist-type.html#deprecated-parameters

cdist/conf/type/__debconf_set_selections/parameter/deprecated/file

@@ -0,0 +1 @@
+'file' has been deprecated in favour of 'line' in order to provide idempotency.