deps: Update cert manager to 1.12.6

[3u13r]

Context

Proposed change(s)

• Bump cert manager to 1.12.6 (see `generateCertManager.sh)

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• Any additional information or context

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[daniel-weisse]

Why are we upgrading to v1.12 instead of v1.13?
Are there any compatibility issues with that version?

[3u13r]

Why are we upgrading to v1.12 instead of v1.13?
Are there any compatibility issues with that version?

From https://cert-manager.io/docs/releases/upgrading/upgrading-1.12-1.13:

IMPORTANT NOTE: If upgrading from a version below v1.12, upgrade to the latest v1.12 release 
before upgrading to v1.13. Otherwise, some certificates may be unexpectedly re-issued 
(see https://github.com/cert-manager/cert-manager/issues/6494#issuecomment-1816112309)

[daniel-weisse]

Can you run the upgrade test and make sure things still work?
lgtm otherwise

[3u13r]

Azure, e2e upgrade, 3:1 : https://github.com/edgelesssys/constellation/actions/runs/7222597118

[github-actions]

Coverage report

Package	Old	New	Trend
internal/constellation/helm	52.70%	52.70%	↔️