Handle release dates after the 22nd

When obtaining a version for a release date in ReleaseManagers::Schedule#version_for_date, we now handle dates that are after the 22nd of a month. Prior to these changes, requesting the version for (for example) the 23rd would produce a version of that month, instead of producing a version for the next month. This commit fixes this problem, effectively treating the 22nd as the last day of a month.

This fixes gitlab-com/gl-infra/delivery#821 (closed)