Release 6.6.0 requires Swift 5.3 and therefore Xcode 12.x

[mluisbrown]

The code added (by me 🙈) in #810 requires Swift 5.3 because of SE-267.

This is because we are using a where clause without the function itself being explicitly generic (even though it is in a generic context):

ReactiveSwift/Sources/SignalProducer.swift

Lines 3098 to 3101 in fe7b02e

	public static func interval(
	_ interval: DispatchTimeInterval,
	on scheduler: DateScheduler
	) -> SignalProducer where Value == Int {

Unsure about whether this is an issue or not, but the README has the Swift 5.1 badge, so I guess it should be fixed. It's also an easy fix. Might be worth considering also using Xcode 11.7 in CI, which would have caught this problem.