Update release notes for shenandoah-8u392-b01.
Sync the copy of the portable specfile with the latest update
Update generate_tarball.sh to be closer to upstream vanilla script inc. no more ECC removal
Update bug URL for RHEL to point to the Red Hat customer portal
Change top_level_dir_name to use the VCS tag, matching new upstream release style tarball
Related: RHEL-12209
