feat: Swift 6 support and switch to swift-testing

[drmohundro]

At least relates to #281.

The bulk of this work is actually switching to swift-testing.

Questions to consider (this list may grow):

• [x] What is the best approach for getting Swift 6 in the build? Still using swiftenv or is there a better approach now?
  • [x] I can just use a matrix build and specify the image that has Swift 6 in it... pretty easy.
  • [x] Though I'm unsure about the MacOS side yet... probably just need to wait for it to release
    • I was able to solve this by using maxim-lobanov/setup-xcode@v1
• [x] I'm not a fan of the name of XMLElementSendable... but it has to have a name
• [x] CHANGELOG