Drop helper `timeSpanFuture'

refs #6778
Drop DateTimeRenderer::timeSpanFuture()

refs #6778
Rename DateTimeRenderer::timePointFuture() -> ...::timeUntil()

refs #6778
Rename DateTimeRenderer::timePointPast() -> ...::timeAgo()

refs #6778
Rename DateTimeRenderer::timeSpanPast() -> ...::timeSince()

refs #6778