Feat: Handle state change lock in prepare downscale

[Segflow]

What this PR does / why we need it:

This PR is part of proposal 4 described in this design document.

PR grafana/dskit#799 adds the ability to lock state change, this PR updates the prepare downscale handler to return 409 if the state change is locked.

We also have a PR in rollout-operator that properly handles this status code: grafana/rollout-operator#343

Which issue(s) this PR fixes:
Fixes

Special notes for your reviewer:

This PR is marked as draft because we need grafana/dskit#799 to be merged first and update go.mod to latest dskit.

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR