docs: add K8s 1.22 compatibility

[nbusseneau]

Following merge of #16989, we now support K8s 1.22 and the default K8s version for Vagrant VMs is 1.21.

[nbusseneau]

@jrajahalme istio.rst is another place in the docs where we discuss Cilium compatibility with K8s, however since this seems like a note specific to testing the Istio integration, I have not edited it:

.. note::

   Cilium integration, as presented in this Getting Started Guide, has
   been tested with Kubernetes releases 1.17, 1.18, 1.19, 1.20 and 1.21.
   This Istio release does not work with Kubernetes 1.16 or older.

Please tell if you'd like me to edit as well :)

[jrajahalme]

@jrajahalme istio.rst is another place in the docs where we discuss Cilium compatibility with K8s, however since this seems like a note specific to testing the Istio integration, I have not edited it:

.. note::

   Cilium integration, as presented in this Getting Started Guide, has
   been tested with Kubernetes releases 1.17, 1.18, 1.19, 1.20 and 1.21.
   This Istio release does not work with Kubernetes 1.16 or older.

Please tell if you'd like me to edit as well :)

If our CI Istio test passes with the new k8s version then the above comment should be updated as well. Same for dropping test coverage for older k8s versions :-)

[nbusseneau]

If our CI Istio test passes with the new k8s version then the above comment should be updated as well. Same for dropping test coverage for older k8s versions :-)

I didn't know the Istio test was part of the regular Jenkins suite. Added! :D