9.12 release notes: NamedDefault semantic change

This MR documents in the 9.12 release notes a semantic change in GHC 9.12 that takes effect as part of the implementation of the NamedDefaults extension. Defaulting will now take place in more situations than it did before, even when the extension is not enabled!

This MR separate from !14057 (closed), as it is only relevant to the 9.12 branch. The changes in !14057 (closed) makes sense both for 9.12 and HEAD, and will be backported to the 9.12 branch separately.