Improve default machine image selection for worker pool having machine of CPU architecture arm64

[acumino]

How to categorize this PR?

/area delivery
/kind enhancement

What this PR does / why we need it:
This PR improves default machine image selection for worker pool having machine of CPU architecture arm64. Also improves the validation message for machine type and machine image of shoot.

Which issue(s) this PR fixes:
Part of https://github.com/gardener/backlog/issues/19

Special notes for your reviewer:
/cc @rfranzke

Release note:

The machine image defaulting does now work based on the CPU architecture of the machine in a given worker pool.

[ialidzhikov]

Improve default image selection for arm worker pool.

Can you elaborate in the PR description which case exactly gets improved and how? Doesn't this PR deserve a release note?

[rfranzke]

I think one main goal of the issue was to provide helpful error messages in case a machine image or type is selected which does not match the architecture in the worker pool, something like this:

gardener/plugin/pkg/shoot/validator/admission.go

Line 1244 in 8a6dce8

detail := fmt.Sprintf("machine image '%s@%s' does not support container runtime '%s', supported values: %+v", worker.Machine.Image.Name, worker.Machine.Image.Version, cr.Type, validValues)

Is this achieved?
What is this PR doing actually? The PR description is pretty poor.

[rfranzke]

/assign @shafeeqes @rfranzke

[rfranzke]

Can you add a release note stating that the machine image defaulting does now work based on the CPU architecture?

[shafeeqes]

Thanks, tested the changes locally and it works as expected.
/lgtm

[rfranzke]

/approve

[gardener-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rfranzke

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