Switch doc generation to use 6.0 (#470)

NB: we always build docs with the latest _release_ version of Swift by
default. So once Swift 6 is released this change will be redundant and
moreover would pin doc gen to 6.0 even once 6.1 released and should be
reverted by then.

Tested to be working ok with our doc gen mechanism:

![CleanShot 2024-06-12 at 14 39
35@2x](https://github.com/apple/swift-testing/assets/65520/07bcefcb-d7a2-4fea-b22d-193bb8198852)

Author: finestructure

.spi.yml

@@ -3,5 +3,8 @@ metadata:
   authors: Apple Inc.
 builder:
   configs:
-    - documentation_targets: [Testing]
+    - swift_version: 5.10
+      scheme: Testing
+    - swift_version: 6.0
+      documentation_targets: [Testing]
       scheme: Testing