azure-pipelines: use 10.14 (and Swift)

[amyspark]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

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(Followup to #6062)

Alternative to #6061: there is now a macOS-10.14 image at Azure pipelines:

• https://github.com/Microsoft/azure-pipelines-image-generation/blob/master/images/macos/macos-10.14-Readme.md

Per #6061 (comment), switch brew's azure script and the script generated by tap-new to use 10.14 instead of 10.13.

(@amyspark here) Also switch to Swift for trashing files in Cask, as AppleScript is currently timing out in CI.

[amyspark]

FTR, I had to rebase the branch to trigger CI.

[scpeters]

well done!

[MikeMcQuaid]

Great work again @amyspark and thanks to you too @scpeters! Could one of you submit a PR to Homebrew/homebrew-test-bot to do the same?