specify opa docker tag in tests

It was defaulting to v1, which is not supported by opa-java-client

Author: NielsCW

opa-async-java-client/src/test/java/com/contentgrid/opa/client/OpaClientIntegrationTests.java

@@ -40,7 +40,7 @@
 @Testcontainers
 class OpaClientIntegrationTests {
 
-    private static final DockerImageName OPA_IMAGE = DockerImageName.parse("openpolicyagent/opa");
+    private static final DockerImageName OPA_IMAGE = DockerImageName.parse("openpolicyagent/opa:0.70.0");
     private static final int OPA_EXPOSED_PORT = 8181;
 
     @Container