Set priority class for kube-scheduler and cluster-autoscaler

[ialidzhikov]

How to categorize this PR?

/area control-plane
/kind bug

What this PR does / why we need it:
See #5634 (comment)

Which issue(s) this PR fixes:
Part of #5634

Release note:

`kube-scheduler` and `cluster-autoscaler` Pods now run with the appropriate priority set according to the following [document](https://github.com/gardener/gardener/blob/v1.57.1/docs/development/priority-classes.md). Previously these Pods were running without a priority class set and were preempted in favour of less important Pods.

[rfranzke]

/lgtm
/approve
/retest

[gardener-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rfranzke

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[gardener-prow]

@ialidzhikov: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-gardener-e2e-kind-ha-multi-zone	1964926	link	false	/test pull-gardener-e2e-kind-ha-multi-zone
pull-gardener-check-vulnerabilities	1964926	link	false	/test pull-gardener-check-vulnerabilities

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