Support for Kubernetes v1.20

[rfranzke]

How to categorize this PR?

/area open-source usability
/kind enhancement
/priority normal
/merge keep-commits
/exp intermediate
/topology garden seed shoot

What this PR does / why we need it:
Support for Kubernetes v1.20

Which issue(s) this PR fixes:
Part of #3260

Special notes for your reviewer:

• I didn't vendor new k8s.io/* Golang sources yet - @timebertt plans to update to kubernetes-v1.19 as part of ☂️-Issue for upgrading to k/*@v0.19 and controller-runtime@v0.7 #3109.
• ⚠️ This PR only contains the Gardener part for supporting 1.20 - there will/must be follow-up PRs for each extension repository where individual support gets added.

Release note:

Gardener can now support shoot clusters with Kubernetes version 1.20. In order to allow creation/update of 1.20 clusters you will have to update the version of your provider extension(s) to a version that supports 1.20 as well. Please consult the respective releases and notes in the provider extension's repository.

[ialidzhikov]

Apart from the failed verify step, it looks very good to me. I played with the PR locally extensively (seed bootstrap for v1.20; hibernation and wake up with v1.20; dynamic volume provisioning with CSI; worker pool with CRI; switch between IPVS and IPTables; worker with suse-chost) and it worked as expected.

/lgtm

[rfranzke]

@ialidzhikov I've pushed a new commit 588143d which fixes the verify step, PTAL.

I played with the PR locally extensively (seed bootstrap for v1.20; hibernation and wake up with v1.20; dynamic volume provisioning with CSI; worker pool with CRI; switch between IPVS and IPTables; worker with suse-chost) and it worked as expected.

That's awesome, thank you very much for your time and efforts and extensive testing, highly appreciated. 👏

[ialidzhikov]

/lgtm

[timuthy]

/lgtm
Nice job! 👏

[rfranzke]

Thanks for your reviews, guys! I'm going ahead merging this now and marking the respective PRs on the provider extensions as ready for review.