Adjust the alert rule name from store to tikv

[Oreoxmt]

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• This PR is translated from: https://github.com/pingcap/docs/pull/10638
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[rleungx]

Why change it? The store includes both TiKV and TiFlash

[Oreoxmt]

Why change it? The store includes both TiKV and TiFlash

@rleungx The issue is https://github.com/pingcap/docs/issues/10626 "Alert name should be PD_cluster_down_tikv_nums rather than PD_cluster_down_store_nums". The alert rule in pd.rules.yml is PD_cluster_down_tikv_nums.