Sync cluster CA to ConfigMap and use it in shoots/{admin,viewer}kubeconfig subresource

[petersutter]

How to categorize this PR?

/area usability
/area security
/kind enhancement

What this PR does / why we need it:
With this PR the cluster CA is synced to a <shoot-name>.ca-cluster ConfigMap in the project namespace of the garden cluster. The shoots/{admin,viewer}kubeconfig subresource retrieves it from there instead of using the Secret resource.
It falls back to the Secret if the ConfigMap has not been synced yet.

Which issue(s) this PR fixes:
Fixes #9091

Special notes for your reviewer:

Release note:

The util function `pkg/utils/gardener.ComputeShootProjectSecretName` has been renamed to `pkg/utils/gardener.ComputeShootProjectResourceName`.

The shoot cluster CA bundle is now stored in a `ConfigMap` in the project namespace of the garden cluster, in addition to storing it in a `Secret`. This `ConfigMap` shares the same name as the pre-existing `Secret`, which is `<shoot-name>.ca-cluster`. The `Secret` will be removed in a future Gardener release. Therefore, if your tooling relies on this `Secret`, you should update it to fetch the `ConfigMap` instead.

[rfranzke]

Impressive PR @petersutter, nice! Thanks for the great commit structure!

Could you please also adapt the occurrences in test/e2e (example)?

[rfranzke]

Thanks, let's get this merged after you rebased and the tests are fixed.

[gardener-prow]

@petersutter: 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-gardener-apidiff	8f9f531	link	false	/test pull-gardener-apidiff

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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[petersutter]

/retest-required

[rfranzke]

/lgtm
/approve

[gardener-prow]

LGTM label has been added.

Git tree hash: fb80533f2df808c1516752ee9233ce83c819b6ae

[gardener-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rfranzke

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