ApplicationNameConverter is still referenced in Logback's defaults

`ApplicationNameConverter` is deprecated since 3.4 and removed in 4.0 but we've overlooked the reference in `org/springframework/boot/logging/logback/defaults.xml`

This produces a failure when our AOT contribution tries to build a model for the XML configuration as the class no longer exists.

(Flagging it as a task as the offending code isn't released in any milestone yet).