Update dependency org.codehaus.mojo:exec-maven-plugin to v3

Author: renovatebot-confluentinc[bot]

pom.xml

@@ -58,7 +58,7 @@
         <easymock.version>5.2.0</easymock.version>
         <!-- keep exec-maven-plugin on 1.5.0 until https://github.com/mojohaus/exec-maven-plugin/issues/76 is fixed
              running our LicenseFinder plugin in create-licenses-for-docker breaks when upgrading to 1.6.0 -->
-        <exec-maven-plugin.version>1.5.0</exec-maven-plugin.version>
+        <exec-maven-plugin.version>3.4.1</exec-maven-plugin.version>
         <spotbugs.version>4.7.1</spotbugs.version>
         <spotbugs.maven.plugin.version>4.7.1.0</spotbugs.maven.plugin.version>
         <java.version>8</java.version>