selftest-ed: Upgrade to version 1.21/1.20.2

selftest-ed: Upgrade to version 1.21/1.20.2

Older versions do not compile with GCC 15. We had different versions to test
GPLv3 exclusion.

1.21 compiles with gcc 15 and 1.20.2 has a CFLAGS tweak to allow it to work
with gcc 15 too.

1.21 is licensed under GPL-2.0 so we need to rework some of the tests.

Tweak the gplv3 test by adding a special override of LICENSE to then test
the license exclusion code.

Modify the archiver selftests to use the new version.

Based on a patch from Khem Raj <raj.khem@gmail.com>

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>