Drop support for older OSes, Xcode versions

[younata]

Similar to Quick/Quick#1146, let's drop support of older OS versions to match what Quick will support.

Quick will be setting the minimum supported versions to Xcode 13, macOS 10.15, iOS 13, tvOS 13, and Swift 5.6. Let's set Nimble to those same versions.

Remember to update:

• Package.swift
• Nimble.podspec
• The GitHub actions workflows
• Nimble.xcodeproj
• For Linux/Swift 5.6: Update the LinuxMain.swift (See the bottom half of this comment detailing why)
• Any relevant documentation