[6.0] Add swift-foundation/swift-foundation-icu branch to release/6.0 checkout config (#75338)

Author: jmschonfeld

utils/update_checkout/update-checkout-config.json

@@ -181,6 +181,8 @@
                 "swift-stress-tester": "release/6.0",
                 "swift-corelibs-xctest": "release/6.0",
                 "swift-corelibs-foundation": "release/6.0",
+                "swift-foundation": "main",
+                "swift-foundation-icu": "main",
                 "swift-corelibs-libdispatch": "release/6.0",
                 "swift-integration-tests": "release/6.0",
                 "swift-xcode-playground-support": "release/6.0",