Add -XDaddTypeAnnotationsToSymbol=true for Error Prone 2.45.0+ JDK 21 compatibility

[Copilot]

Error Prone 2.45.0+ requires the -XDaddTypeAnnotationsToSymbol=true compiler flag when running on JDK 21. Without it, compilation fails with:

> -XDaddTypeAnnotationsToSymbol=true is required by Error Prone on JDK 21

Changes

• Added required compiler flag to ai.errorprone-conventions.gradle.kts for all JavaCompile tasks
• Flag is unconditionally set since the project uses JDK 21 toolchain and targets Java 8

This unblocks the Error Prone 2.45.0 → 2.46.0 upgrade in #4556.

tasks {
  withType<JavaCompile>().configureEach {
    with(options) {
      // Error Prone 2.45.0+ requires this flag when running on JDK 21
      compilerArgs.add("-XDaddTypeAnnotationsToSymbol=true")
      
      errorprone {
        // ... existing config
      }
    }
  }
}

Original prompt

Fix the GitHub Action failures in #4556

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