Talk less about "Single UNIX" because it has a confusing numbering

Talk less about "Single UNIX" because it has a confusing numbering
scheme.  Instead, stick to the XPG/POSIX numbering scheme that has
clearly won: earlier this year, POSIX Issue 8 has been released,
and nobody calls it "Single UNIX version 5".

The XPG/POSIX numbering scheme is also preferable because it can
be used for the entire history leading up to the current POSIX
standard (1988-2024), whereas the Single UNIX numbering scheme
only works for the period from XPG4.2 to POSIX Issue 7 (1994-2008).

This patch mostly changes (informational) paragraph headers
and corrects two instances of the incorrect term "XPG version"
to the correct term "XPG Issue".  The (normative) .St argument
definitions remain unchanged, and all supported .St arguments
remain documented, including the deprecated arguments -susv1 to -susv4.

No objections when shown on tech@ a few months ago.