GHA: canary-build.yml add Gradle 8.7 to matrix.

This is due to API removal preventing build against Gradle 8.8

org/unbrokendome/gradle/plugins/xjc/testutil/GradleTestUtils.kt:23:49 Unresolved reference: start

This API looks only used for integration testing which I'd hope
CI-Canary-Consume will demonstrate.

.github/workflows/canary-build.yml

@@ -39,6 +39,7 @@ jobs:
         gradle: [
           'latest',
           '8',
+          '8.7',  # BuildOperationExecutor#start() removal in 8.8
           '8.5',  # JDK21
           '7',
           '7.6',