Add gardener.cloud/purpose: kube-system label to all seeds

[bd3lage]

How to categorize this PR?

/area control-plane
/kind task

What this PR does / why we need it:

Make sure ALL seeds have the label gardener.cloud/purpose: kube-system for kube-system namespace
(to prevent our webhooks that already exclude above label from interfering; our managed seeds have this label automatically but unmanaged seeds not)

Which issue(s) this PR fixes:
Fixes landscape-stability issue#39

Special notes for your reviewer:

Release note:

The `gardener.cloud/purpose: kube-system` label is now added to the `kube-system` namespace by the `gardenlet'`s `Seed` controller.

[gardener-prow]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[rfranzke]

Why is this PR still in draft mode?

[gardener-prow]

@bd3lage: 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-dependency-watchdog-verify-image-build	779a6e6	link	true	/test pull-dependency-watchdog-verify-image-build
pull-gardener-check-vulnerabilities	da96e86	link	false	/test pull-gardener-check-vulnerabilities

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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[rfranzke]

/lgtm
/approve

[gardener-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rfranzke

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