DfsGarbageCollector: #setReflogExpire with Instant instead of Date

DfsGarbageCollector: #setReflogExpire with Instant instead of Date

The Date API is full of major design flaws and pitfalls and should be
avoided at all costs. Prefer the java.time APIs, specifically,
java.time.Instant (for physical time) and java.time.LocalDate[Time]
(for civil time). [1]

Replace the Date with Instant in the
DfsGarbageCollector#setReflogExpire method.

[1] https://errorprone.info/bugpattern/JavaUtilDate

Change-Id: Ie98e426e63621e8bef96c31bca56aec0c8eef5a6