WIP: Use CA in cloudprofile for STACKIT clients

[nschad]

How to categorize this PR?

/kind enhancement

What this PR does / why we need it:

Removes the custom-ca secret and instead uses the CA from the Cloudprofile

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Breaking changes:

[ske-prow]

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[ske-prow]

@nschad: 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-extension-provider-stackit-integration-tests	ec54626	link	true	/test pull-gardener-extension-provider-stackit-integration-tests

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 the gardener testing guideline for how to avoid and hunt flakes.

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