Fix the backup.gardener.cloud/created-by annotation not being added on existing etcd-backup Secrets

[ialidzhikov]

How to categorize this PR?

/area quality
/kind bug

What this PR does / why we need it:
In

gardener/extensions/pkg/controller/backupentry/genericactuator/actuator.go

Lines 91 to 97 in 6988da8

	etcdSecret := emptyEtcdBackupSecret(be.Name)
	metav1.SetMetaDataAnnotation(&etcdSecret.ObjectMeta, AnnotationKeyCreatedByBackupEntry, be.Name)
	_, err = controllerutils.GetAndCreateOrMergePatch(ctx, a.client, etcdSecret, func() error {
	etcdSecret.Data = etcdSecretData
	return nil
	})

the corresponding annotation will be set for new etcd-backup Secret but it won't be if the Secret already exists in the system.

We also added logs around the etcd-backup Secret deletion for debugging purposes as we faced another occurrences of #8675.

Which issue(s) this PR fixes:
Follow-up on #8675

Special notes for your reviewer:
N/A

Release note:

extension library: An issue causing the `backup.gardener.cloud/created-by` annotation not being added on existing `etcd-backup` Secrets is now fixed.

[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

[ialidzhikov]

/cc @Kostov6

[rfranzke]

/lgtm
/approve
/retest

Do we need to cherry-pick this? 👀

[gardener-prow]

LGTM label has been added.

Git tree hash: d2ee1a31e84031eb18c51ee61b67248fc582143e

[gardener-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rfranzke

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[ialidzhikov]

Do we need to cherry-pick this? 👀

IMO, it is not needed.
Context: we looked into a new occurrence of #8675. It was a new Shoot creation failed due to a missing etcd-backup Secret. On theory, the fix from #8709 should be good enough to prevent new occurrences because the annotation is added a new etcd-backup Secrets. But we were not able to find how it happened.
While looking into it, we noticed that the annotation is not added on existing etcd-backup Secrets (created prior to #8709). We also added logs for the etcd-backup Secret deletion so that we have more info if/when it happens again. It was only one occurrence of #8675 in the last 5months so it is not that pressing IMO.