feat: Handle state change lock in prepare downscale

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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

Notes to reviewer:

dskit #759 made it not possible to use a zero heartbeat-period or zero heartbeat-timeout in the ring configuration.

I had to update some pkg/ruler and pkg/limits tests to ensure both these fields are set otherwise the test fails.

We also removed 0 = disabled from our configuration documents. We do not use it and we believe that should be an incorrect configuration.

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• 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

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