readme: set latest release to 6.3.0
readme: set latest release to 6.3.0

Signed-off-by: Radostin Stoyanov <af191057dab5b67c1c1061d181b58b884c4ed7f3@fedoraproject.org>

checkpoint-restore/go-criu,checkpoint-restore/go-criu,checkpoint-restore/go-criu