build(deps): update dependency org.apache.maven.plugins:maven-compile…

…r-plugin to v3.9.0 (#974)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
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google-cloud-pubsub/pom.xml

@@ -159,7 +159,7 @@
     <plugins>
       <plugin>
         <artifactId>maven-compiler-plugin</artifactId>
-        <version>3.8.1</version>
+        <version>3.9.0</version>
         <configuration>
           <source>1.8</source>
           <target>1.8</target>