Update plugin properties to declare them as multi-values property

[ianwilliams1]

SonarQube LTS 8.9.0
sonar-groovy-plugin-1.7

Plugin is throwing the following WARNING:

WARN: Property 'sonar.groovy.file.suffixes' is not declared as multi-values/property set but was read using 'getStringArray' method. The SonarQube plugin declaring this property should be updated.

As a result of SonarQube plugin API change, I believe file:
sonar-groovy-plugin/src/main/java/org/sonar/plugins/groovy/GroovyPlugin.java
needs to be updated to include: .multiValues(true)

See similar change for dotnet plugin.
And similar change for Cxx-plugin

This warning is an annoyance as it's an issue thrown by SonarQube in relation to the plugin, yet is displayed to the users who have no idea what it's about.