Replace Go model generation for kubernetes-model-gatewayapi

[manusa]

Description

Part of #6130

Replace model generation for kubernetes-model-gatewayapi module with new OpenAPI approach.

Tasks

• Remove build-helper-maven-plugin
• Remove maven-antrun-plugin from the generate profile
• Add openapi-model-generator-maven-plugin to the generate profile
• Configure the plugin to generate the specific classes
• Remove the go-related files and directories (Makefile, cmd)
• Remove invocation from the generateModel.sh script

[manusa]

This model defines another ObjectReference type that collides with the one from API Machinery.
Things need to be reworked to avoid the problem.