[v10.0.x] Alerting: Convert 'Both' type Prometheus queries to 'Range' in migration

[grafana-delivery-bot]

Backport 00d5f7f from #70781

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What is this feature?

This feature updates the legacy migration to convert Prometheus queries of type Both to type Range and log a warning during migration.

Why do we need this feature?

Legacy supports alerting on Prometheus queries of type Both. This returns a mixed response with two dataframes (TimeSeriesWide & NumericWide), however Server Side Expressions and UA does not currently support this type of mixed response.

Who is this feature for?

Users of legacy alerting who will be migrating to UA.

Which issue(s) does this PR fix?:

Fixes #70768

Special notes for your reviewer:

There is the possibility to better support migrating this functionality by:

• Splitting the query into two: one for instant and one for range.
• Splitting the condition into two: one for each query, separated by OR.

However, relying on a Both query instead of multiple conditions to do this in legacy is likely
to be unintentional. In addition, this would require more robust operator precedence in classic conditions.
Given these reasons, we opt to convert them to Range queries and log a warning.

Please check that:

• It works as expected from a user's perspective.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.