RxSwift 6.0.0 RC2

This is the second and (probably) final Release Candidate of RxSwift 6.
The official 6.0.0 should be released in the upcoming weeks and should be mostly identical to this.

Note: RxSwift 6 supports Xcode 12 and Swift 5.3 and has a minimum deployment target of iOS 9.

Everything in RC1, plus:

• Support for XCFrameworks, with binaries now bundled with each release.
• Updated tests for new Infallible trait.
• Fix an issue building for watchOS.
• Move URLSession logging settings to a more appropriate URLSession.rx.shouldLogRequest configurable closure.
• README, CI, and additional house cleaning.