docs: Update linux distribution compatibility
docs: Update linux distribution compatibility

* CoreOS is End of Life as of 2020-05-22.
* Flatcar succeeds CoreOS for similar use cases and users are using it.
* We had reports from Azure users on 16.04.1 with Linux kernel 4.15.

Signed-off-by: Joe Stringer <16a9a54ddf4259952e3c118c763138e83693d7fd@cilium.io>

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