fix: correct instance label handling for DM Discovery component

[shunki-fujita]

What problem does this PR solve?

The topologySpreadConstraints for the dm-discovery pod were not applied correctly because the instance label used in the constraints did not include the -dm suffix.
This caused a mismatch between the constraint's label selector and the actual pod labels, potentially leading to unintended pod scheduling behavior.

Close #6149

What is changed and how does it work?

The logic that sets the app.kubernetes.io/instance label in topologySpreadConstraints has been updated.
For the dm-discovery component, the instance label now appends a -dm suffix to match the actual label used on the pod.
This ensures that the topology spread constraints work correctly by aligning with the pod's label.

Code changes

• Has Go code change
• Has CI related scripts change

Tests

• Unit test
• E2E test
• Manual test
• No code

Side effects

• Breaking backward compatibility
• Other side effects:

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release Notes

Please refer to Release Notes Language Style Guide before writing the release note.

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[sre-bot]

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[ti-chi-bot]

Welcome @shunki-fujita! It looks like this is your first PR to pingcap/tidb-operator 🎉