Code Monitors: use committer date for after filter

Created by: camdencheek

Previously, we were using the author date to populate the after filter, but our search filters use the committer date. This meant we could make a commit with different committer and author dates such that we would repeatedly return the same result for the generated code monitor queries. This fixes the issue by populating the after filter with committer date rather than author date.

This problem only exists because we limit our searches by date rather than by commit hash. The problem will no longer be possible after repo-aware code monitors are active.

Stacked on #30056