repos: change security repo link for future proofing and add note for bullseye

repos: change security repo link for future proofing and add note for bullseye

Actual this is a no-change change, as they URLs both resolve to the
exact same folders (are symlinked), i.e., see:

http://security-cdn.debian.org/dists/
vs.
http://security-cdn.debian.org/debian-security/dists/

but just change it nonetheless to match Debian's recommendation.
Also, add a FIXME not for the future 7.0 as there we need to change
the security suite name too[0].

[0]: https://lists.debian.org/debian-devel-announce/2019/07/msg00004.html

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>