Add subgroup event support and update pom.xml

[n0kke]

Testing done

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[krisstern]

Hi @n0kke may I ask what is this PR intended for?

[n0kke]

Hi @n0kke may I ask what is this PR intended for?

The purpose of this PR is to add support for GitLab subgroup webhook events, which are not currently handled by the plugin (along with other event types).

This change enables Jenkins jobs to respond correctly to events originating from GitLab subgroups, aligning their behaviour with that of group and project events.

The update to the pom.xml was necessary solely to resolve build and dependency compatibility issues encountered during the implementation and testing of this change, and does not affect the plugin's runtime behaviour.