Issue alerts based on releases don't work well for mobile

### Problem Statement

The alert rules only let you filter based on the latest release in a given environment. However, mobile release processes tend to have multiple in flight at the same time. It would be nice if there was an option to have an alert rule that only filtered for releases < a certain age (like 2 weeks or 3 weeks). Filtering on the latest release only makes the feature unusable for mobile because a release gets created but doesn't roll out for a week.

### Solution Brainstorm

1. Allow a rule to be the most recent X releases
2. Allow a rule to be the releases X% of sessions
Something else along those lines

### Product Area

Alerts

The issue names only the Alerts product area and no files, tests, or entry points. Start by locating the existing release-based alert rules, then resolve the intended filtering semantics with maintainers; completion criteria are not specified beyond making mobile release alerts usable.

Written by the indexing model from the issue text.