debian: Build-Depend on g++ 11 or greater

debian: Build-Depend on g++ 11 or greater

Rely on the packaging system to provide a suitable g++ of version 11
or greater, and removing the corresponding hard-coding from
debian/rules, since cmake will then find a suitable version. This
seems better than trying to hard-code a particular version in
debian/rules, and Debian package building tools like e.g. sbuild will
then do the right thing.

This enables Reef (v18.2.0) to build on Debian bookworm in a clean
chroot.

Fixes: https://tracker.ceph.com/issues/61845
Signed-off-by: Matthew Vernon <mvernon@wikimedia.org>