Bump netty-all from 4.1.12.Final to 4.1.42.Final in /core

Bumps [netty-all](https://github.com/netty/netty) from 4.1.12.Final to 4.1.42.Final.
- [Release notes](https://github.com/netty/netty/releases)
- [Commits](netty/netty@netty-4.1.12.Final...netty-4.1.42.Final)

Fixes #1373

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=274802452

core/pom.xml

@@ -304,7 +304,7 @@
       <!-- Apache 2.0 -->
       <groupId>io.netty</groupId>
       <artifactId>netty-all</artifactId>
-      <version>4.1.12.Final</version>
+      <version>4.1.42.Final</version>
       <scope>test</scope>
     </dependency>
     <dependency>