fix: deriving legacy ranges

[briangregoryholmes]

• Fixes an issue where legacy rill- prefixed time ranges had an undefined range grain when parsed and were not being factored into the derived aggregation grain
• Fixes an issue where legacy rill- prefixed ranges were not snapping to the day boundary

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

[AdityaHegde]

@begelundmuller this leads to legacy ISO from alerts/reports to have different meaning. So will leave it up to you to approve this.

Ignore, old alerts/reports go through a different code path: https://github.com/rilldata/rill/blob/main/runtime/metricsview/executor/executor_rewrite_time.go#L68
https://github.com/rilldata/rill/blob/main/runtime/metricsview/executor/executor_rewrite_time.go#L115

So this will only affect calls from UI for now.

[briangregoryholmes]

Removed the request for Benjamin's review.