During legacy migration every migrated rule was given a label rule_uid=<uid>.
This was used to silence DatasourceError/DatasourceNoData alerts for
migrated rules that had either ExecutionErrorState/NoDataState set to
keep_state, respectively.

This could potentially create a large amount of silences and a high cardinality
label. Both of these scenarios have poor outcomes for CPU load and latency in
unified alerting.

Instead, this change creates one label per ExecutionErrorState/NoDataState when
they are set to keep_state as well as two silence rules, if rules with said
labels were created during migration. These silence rules are:

- __legacy_silence_error_keep_state__ = true
- __legacy_silence_nodata_keep_state__ = true

This will drastically reduce the number of created silence rules in most cases
as well as not create the potentially high cardinality label `rule_uid`.