Consider moving up to Swift 5.0 as the minimum compiler and language versions

[thomasvl]

We currently require a 4.2 or later compiler and support the language version of 4.2 or later.

According to https://www.swift.org/download/, 4.2 came out with Xcode 10.0 on September 17, 2018; 4.2.1 was Xcode 10.1 on October 30, 2018. 4.2.2 & 4.2.3 were linux only, with the later being on February 28, 2019. 4.2.4 was on March 29, 2019 and is Ubuntu 16.04 only.

There is specific support for 4.2 in the library and generator, so it would simply things. The AppStore won't accept things off that older Xcode version, but in theory , someone could be using a newer compiler but still compiling the generated code in the 4.2 mode.

Swift Package Index compatibility reports these days only mention the 5.x version, so that might be a signal that the 4.x support really can be dropped.

Thoughts?

[thomasvl]

We should also revisit https://github.com/apple/swift-protobuf/labels/API%20change for things to do as part of a major version bump.