chore: bump mockgen, proto-gen-go and grpc deps

[idebeijer]

• Upgrade mockgen to v0.6.0 for Go 1.25 compatibility.
• Upgrade protoc-gen-go to v1.35.1 and protoc-gen-go-grpc v1.5.1 to align with oldest supported k/k (1.32). https://github.com/kubernetes/kubernetes/blob/v1.32.12/go.mod#L88
• Bump konnectivity-client Go version to 1.23.
• Remove redundant license header generation for make gen-proto.

This PR is based on #810, that should be merged first.

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