Fix Prometheus alerts using oldstyle label selector

[xrstf]

What this PR does / why we need it:
When moving to the operator, we also switched around the labels. This made the Prometheus alerts misfire. To keep the newly fixed rules compatible with the old Helm chart, I adjusted the chart to also provide the new labels.

Fixes #6152.

Does this PR introduce a user-facing change?:

Fix Prometheus alerts misfiring about absent KKP components

[kubermatic-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xrstf

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[xrstf]

/cherrypick release/v2.15

[kubermatic-bot]

@xrstf: once the present PR merges, I will cherry-pick it on top of release/v2.15 in a new PR and assign it to you.

In response to this:

/cherrypick release/v2.15

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[youssefazrak]

/lgtm

[kubermatic-bot]

LGTM label has been added.

Git tree hash: 67bd100a853e9b00f2a06ed9f9cfb49340cc198e

[youssefazrak]

/lgtm

[kubermatic-bot]

LGTM label has been added.

Git tree hash: 596f3951d5e3aa6a096e798bb9f74c04ea7bc974

[youssefazrak]

/retest

[kubermatic-bot]

@xrstf: #6167 failed to apply on top of branch "release/v2.15":

Applying: Fix Prometheus alerts using oldstyle label selector
Using index info to reconstruct a base tree...
M	charts/monitoring/prometheus/Chart.yaml
Falling back to patching base and 3-way merge...
Auto-merging charts/monitoring/prometheus/Chart.yaml
CONFLICT (content): Merge conflict in charts/monitoring/prometheus/Chart.yaml
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 Fix Prometheus alerts using oldstyle label selector
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

In response to this:

/cherrypick release/v2.15

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.