[admission] Add priorityClassName

[timuthy]

How to categorize this PR?

/area robustness
/area networking
/kind enhancement

What this PR does / why we need it:
This PR enhances the admission chart by the option to set a priorityClassName.

Which issue(s) this PR fixes:
Part of gardener/gardener#9936.

Release note:

A `priorityClassName` can now be set for the admission deployment via the `gardener-extension-admission-cilium` Helm chart.

[gardener-prow]

@timuthy: The following test 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-extension-networking-cilium-e2e-kind	b1d210e	link	true	/test pull-extension-networking-cilium-e2e-kind

Full PR test history. Your PR dashboard. Command help for this repository.
Please help us cut down on flakes by linking this test failure to an open flake report or filing a new flake report if you can't find an existing one. Also see our testing guideline for how to avoid and hunt flakes.

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[axel7born]

/lgtm