textproc/saxon-he: update to 12.8, for OpenJDK 8

Saxon-HE 12.6 did not run on OpenJDK 8, but required OpenJDK 11.
This was reverted in 12.7 with a warning that Saxon 13 will
probably not support Java 8 any more.

While here, update all the way to 12.8.

While here, also append a post-stage smoke test that prints the
first line of the help, which would fail on 12.6 to prevent
packing when it doesn't work with the default java version.

The "security gap" fixed in 12.8 only applies to the Professional
and Enterprise Editions (PE, EE) not to this Home Edition (HE):
https://blog.saxonica.com/announcements/2025/07/saxon-11.7-and-12.8.html

Changelog: https://blog.saxonica.com/announcements/2025/05/saxon-12.7.html
Changelog: https://blog.saxonica.com/announcements/2025/07/saxon-12.8.html

Approved by: portmgr (just-fix-it blanket approval)
MFH: 2025Q3