[gdb/testsuite] Fix gdb.python/py-thread-exited.exp

Two fixes in gdb.python/py-thread-exited.exp:
- fix the copyright notice validity range (PR testsuite/30687):
  2022-202 -> 2022-2023, and
- add missing "require allow_python_tests".

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30687