Increase mas's minimum supported macOS version from 10.13 to 10.15 or newer

[rgoldberg]

Increase mas's minimum supported macOS version from 10.13 to 10.15 or newer.

This should be done only on the release of a major version.

The user base on macOS 10.13 & 10.14 is probably small.

The benefits of which I'm aware of increasing the minimum macOS version for running mas are (by minimum version):

• 10.15
  • Swift Concurrency
  • swift-testing
  • Nimble 11+ (currently 13.x)
  • Quick 6+ (currently 7.x)
  • NSWorkspace.openApplication(…)
  • JSONSerialization.WritingOptions.withoutEscapingSlashes
• 11
  • Logger
• 12
  • Date.ISO8601Format
  • Date.ISO8601FormatStyle
• 13
  • Regex & related
  • Locale.Currency
• 15
  • RangeSet

I know that there are other benefits, I just haven't yet identified them.