docs: bump minimum required clang version for development to 7.0
docs: bump minimum required clang version for development to 7.0

To compile the inline assembly in bpf/include/bpf/ctx/xdp.h clang 7.0 or
higher is required. As it may only be needed for the local make, but not
at runtime if e.g. XDP isn't used, only bump the version in development
requirements for now. In the installation requirements keep clang 5.0 as
a minimum requirement with clang 7.0 stated as the recommended version.

Fixes #10523

Signed-off-by: Tobias Klauser <9249c549618448e4c699e8032ba6c7f0d7fd4b7f@distanz.ch>

tgraf/cilium,tgraf/cilium,tklauser/cilium,tgraf/cilium,cilium-team/cilium,cilium/cilium,tklauser/cilium,tgraf/cilium,cilium/cilium,tklauser/cilium,cilium-team/cilium,tgraf/cilium,cilium/cilium,michi-covalent/cilium,cilium/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium/cilium,tklauser/cilium,michi-covalent/cilium,tgraf/cilium,tklauser/cilium,michi-covalent/cilium