docs/rootless.md: drop support for Debian 9 and Fedora 30
docs/rootless.md: drop support for Debian 9 and Fedora 30

Debian 9 has reached EOL on 2020-07-06: https://wiki.debian.org/DebianReleases
Fedora 30 has reached EOL on 2020-05-26: https://fedoraproject.org/wiki/End_of_life

Signed-off-by: Akihiro Suda <10ddef0dea2a66ab193b07198faa879adcb213d8@hco.ntt.co.jp>

tonistiigi/buildkit_poc,tonistiigi/buildkit_poc