Don't consider Crashloopbackoff pods as part of PDB

[himanshu-kun]

How to categorize this issue?

/area performance
/area robustness
/area usability
/kind enhancement
/priority 3

What would you like to be added:
Currently upstream treats Crashloopbackoff pods as Unavailable and so if a PDB is configured with maxUnavailable=1 with 2 pods , 1 pod Pending and other Crashloopbackoff, then the pod eviction request is denied for such pod and node draining can't proceed.
There is a discussion upstream to deal with this kubernetes/kubernetes#72320 and a PR to ignore Crashloopbackoff pods from PDB is raised kubernetes/kubernetes#105296

Testing is required after the PR gets merged and MCM starts using the corresponding k8s version
Why is this needed:
So that MCM draining is not stuck till drainTimeout for CrashLoopbackoff pods with PDB.

[himanshu-kun]

Post grooming discussion

The onus is on the customer now to configure the PDB in a way which allows to drain the CrashLoopBackoff pods also .
This PR introduced spec.unhealthyPodEvictionPolicy recently. It is currently in alpha and needs to be enabled via feature gate PDBUnhealthyPodEvictionPolicy.

We need to update the gardener docs after testing this feature, to tell the customers how to do use this. Also need to update the DOD playbook for operators.

[timuthy]

FYI: gardener/gardener#8821