Documentation: clean up upgrade instructions
Documentation: clean up upgrade instructions

Versions of cilium which are EOL'd (e.g., v1.0-v1.2) have their own
documentation about how to upgrade Cilium. Since we no longer support
or update those versions with backports / documentation changes, we
can remove the upgrade instructions for them from the upgrade guide.
Tell users to refer to the documentation for that version for specific
upgrade information to keep the upgrade guide at a sane size and more
maintainable.

Signed-off by: Ian Vernon <ian@cilium.io>

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