Update ASM version to 9.6 to support JDK 22 class files

This should fix a JDK 22-ea issue with the test infrastructure:

```
java.lang.IllegalArgumentException: Unsupported class file major version 67
	at com.google.turbine.lower.LowerIntegrationTest.test(LowerIntegrationTest.java:432)
```

The javac flag `--enable-preview` is deliberately only supported for the latest language level, so testing preview features (like string templates) on JDK 22 requires using JDK 22 class files.

PiperOrigin-RevId: 602464584

pom.xml

@@ -30,7 +30,7 @@
   <url>https://github.com/google/turbine</url>
 
   <properties>
-    <asm.version>9.5</asm.version>
+    <asm.version>9.6</asm.version>
     <guava.version>32.1.1-jre</guava.version>
     <errorprone.version>2.16</errorprone.version>
     <maven-javadoc-plugin.version>3.3.1</maven-javadoc-plugin.version>