chore: remove hard-coded versions for iam proto/grpc (googleapis#11584)

This is based off description here: googleapis#11557 (comment)

Author: ldetmer

java-iam/google-iam-policy/pom.xml

@@ -56,7 +56,6 @@
     <dependency>
       <groupId>com.google.api.grpc</groupId>
       <artifactId>proto-google-iam-v3beta</artifactId>
-      <version>1.51.0</version>
     </dependency>
     <dependency>
       <groupId>com.google.guava</groupId>
@@ -104,7 +103,6 @@
     <dependency>
       <groupId>com.google.api.grpc</groupId>
       <artifactId>grpc-google-iam-v3beta</artifactId>
-      <version>1.51.0</version>
       <scope>test</scope>
     </dependency>
     <!-- Need testing utility classes for generated gRPC clients tests -->