Switch to use commons-digester3

[harrisric]

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Changed the dependency on commons-digester to use commons-digester3 instead.

I understand that this may be blocked for a later major version change similarly to #53 running japicmp:cmp yields the following output:

[ERROR] Failed to execute goal com.github.siom79.japicmp:japicmp-maven-plugin:0.23.1:cmp (default-cli) on project commons-validator: There is at least one incompatibility: org.apache.commons.digester.ObjectCreationFactory[org.apache.commons.digester.ObjectCreationFactory]:INTERFACE_REMOVED,org.apache.commons.validator.FormSetFactory:SUPERCLASS_REMOVED,org.apache.commons.validator.FormSetFactory:METHOD_ABSTRACT_ADDED_IN_IMPLEMENTED_INTERFACE -> [Help 1]