Dropping swift-syntax-510 support

Howdy, fellow test authors!

Yesterday, my colleague @smontgomery announced our intent to align our first stable release with Swift 6.

In support of that goal, we've been hard at work improving the diagnostics emitted by swift-testing when swift-syntax-600 is available, and we've reached a fork in the road. The code we run when swift-syntax-600 is used is significantly hardier and produces significantly better diagnostics than when swift-syntax-510 is used. Supporting both is no longer viable.

Beginning with our next prerelease tag (which should be 0.8.0), swift-testing will drop support for swift-syntax-510 and will require swift-syntax-600. This PR tracks this source change.