ci: Remove Swift 6.0 from Ubuntu build matrix

Swift 6.0 compiler has a crash bug in TypeChecker::overApproximateAvailabilityAtLocation
that causes builds to fail. This is fixed in Swift 6.1+.

The compiler crash occurs during compilation phase, not during tests.
Keeping Swift 6.1, 6.2, and nightly builds which all pass successfully.

Closes #149

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: leogdion

.github/workflows/BushelKit.yml

@@ -13,7 +13,6 @@ jobs:
     strategy:
       matrix:
         image:
-          - swift:6.0
           - swift:6.1
           - swift:6.2
           - swiftlang/swift:nightly-6.2-noble