gardenlet: Do not deploy blackbox-exporter for Shoots with .spec.purpose=testing

[ialidzhikov]

How to categorize this PR?

/area monitoring
/kind regression

What this PR does / why we need it:
After #9695, Shoots with spec.purpose=testing are unhealthy with:

Deployment "shoot--foo--bar/blackbox-exporter" is unhealthy: condition "Available" has invalid status False (expected True) due to MinimumReplicasUnavailable: Deployment does not have minimum availability.

% k -n shoot--foo--bar get po -l app=blackbox-exporter
NAME                                 READY   STATUS              RESTARTS   AGE
blackbox-exporter-6dd585c5cf-j8z2w   0/1     ContainerCreating   0          21m
blackbox-exporter-6dd585c5cf-k89n8   0/1     ContainerCreating   0          46m

% k -n shoot--foo--bar describe po blackbox-exporter-6dd585c5cf-j8z2w

  Warning  FailedMount       19s (x18 over 20m)  kubelet                MountVolume.SetUp failed for volume "cluster-access" : secret "shoot-access-prometheus-shoot" not found

Which issue(s) this PR fixes:
N/A

Special notes for your reviewer:
N/A

Release note:

gardenlet: An issue causing the blackbox-exporter Deployment to be created and to be unhealthy in the Shoot control plane for Shoots with `.spec.purpose=testing` 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]

/cherry-pick release-v1.95

[gardener-ci-robot]

@ialidzhikov: once the present PR merges, I will cherry-pick it on top of release-v1.95 in a new PR and assign it to you.

In response to this:

/cherry-pick release-v1.95

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[AleksandarSavchev]

Tested it locally.
/lgtm

[gardener-prow]

LGTM label has been added.

Git tree hash: 0e9153c4cd1a44c60261a5ee12d045c9f01ea7d3

[ialidzhikov]

/approve

[gardener-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: AleksandarSavchev, ialidzhikov

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[gardener-ci-robot]

@ialidzhikov: new pull request created: #9798

In response to this:

/cherry-pick release-v1.95

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[rfranzke]

Thanks for this fix @ialidzhikov, appreciated!