Consider returning file creation/modification/access time as an Instant.

[MohammedKHC]

Since Kotlin 2.3.0, Instant is stable.

By changing FileMetadata.createdAtMillis,
FileMetadata.lastModifiedAtMillis, and
FileMetadata.lastAccessedAtMillis to Instant and renaming them to
createdAt, lastModifiedAt, and lastAccessedAt, we avoid losing
nanosecond precision.

We can still provide deprecated extension properties that return the
millisecond value from the corresponding Instant.

[MohammedKHC]

Additionally, I would like to note that the JVM already provides a conversion from FileTime to Java’s Instant, which we can then use to create a Kotlin Instant.

[JakeWharton]

Extensions are insufficient, for what it's worth. We need to maintain both the constructor and members for binary compatibility. Doing this would also be counter to #1690 so we actually need to wait until Kotlin 2.3 is our minimum-supported version.