Mention illumos in the NEWS entry

Oracle Solaris is not affected: its sendfile() reports the number of
transferred bytes correctly.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: serhiy-storchaka

Misc/NEWS.d/next/Library/2026-07-21-08-31-55.gh-issue-154324.vDxV5U.rst

@@ -1,3 +1,3 @@
-Fix :func:`os.sendfile` on Solaris and illumos:
+Fix :func:`os.sendfile` on illumos:
 it no longer reports a successful transfer
 when the underlying system call failed without writing any data.