fix panic on failed secret update on hash migration

[MartinWeindel]

What this PR does / why we need it:
If during certificate hash migration the update of a secret fails, a panic occurs because of overwritten variable obj.

Observed a panic: &runtime.TypeAssertionError{_interface:(*runtime._type)(0x1908a00), concrete:(*runtime._type)(0x1968260), asserted:(*runtime._type)(0x194ff00), missingMethod:""} (interface conversion: abstract.ObjectData is *v1.Secret, not *v1alpha1.Certificate)
...
github.com/gardener/cert-management/pkg/controller/issuer/certificate.(*certReconciler).failed(...)
	/build/pkg/controller/issuer/certificate/reconciler.go:1034
github.com/gardener/cert-management/pkg/controller/issuer/certificate.(*certReconciler).migrateSpecHash(0xc000012380, 0x1bd9d40, 0xc0000a89b0, 0x1bec938, 0xc00068c900, 0xc001100400, 0x38, 0xc001100480, 0x38, 0xc0011004c0, ...)
	/build/pkg/controller/issuer/certificate/reconciler.go:945 +0x60a

As update failure can only result in a new certificate request, this fix does not fail the migration anymore.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

fix TypeAssertionError panic on failed secret update on hash migration