Meta: set version for "build-helper-maven-plugin"

Created by: jmini

This PR removes those warnings

[INFO] Scanning for projects...
[WARNING] 
[WARNING] Some problems were encountered while building the effective model for org.openapitools:myClientCodegen-openapi-generator:jar:1.0.0
[WARNING] 'build.plugins.plugin.version' for org.codehaus.mojo:build-helper-maven-plugin is missing. @ line 65, column 21
[WARNING] 
[WARNING] It is highly recommended to fix these problems because they threaten the stability of your build.
[WARNING] 
[WARNING] For this reason, future Maven versions might no longer support building such malformed projects.
[WARNING] 

That are created when this command is executed:

mvn verify -f samples/meta-codegen/lib/pom.xml

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Corresponding script: bin/meta-codegen.sh