doc: Reference POSIX:2024 for rm -f future requirement.
* NEWS-future: The previously "next" POSIX version is the current
POSIX (2024) now, requiring rm -f to be a no-op.
Rename file to fix copyright update glitch
* Makefile.am (EXTRA_DIST), NEWS: Adjust to file renaming.
* NEWS-future: Rename from NEWS-2.0, so that "make update-copyright"
does not think this is a man page and use \(en rather than - in year
ranges. Change \(en back to -.