Switch gardenlet to logr (3)

[timebertt]

How to categorize this PR?

/area logging dev-productivity
/kind enhancement

What this PR does / why we need it:
Migrate gardenlet's certificate bootstrap and rotation to logr.

Which issue(s) this PR fixes:
Part of #4251

Special notes for your reviewer(s):
✅ Depends on #6259, hence it's in draft state until this PR is merged.

Release note:

NONE

[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]

nice

[rfranzke]

/lgtm
/approve

[gardener-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rfranzke

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[gardener-prow]

@timebertt: 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	2b98a3f	link	false	/test pull-gardener-apidiff

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