Add project_events attribute to gitlab_group_hook resource

What does this MR do and why?

Adds the project_events boolean attribute to the gitlab_group_hook resource.

The GitLab API supports project_events for group webhooks (API docs), but the Terraform provider was missing this field. Users were unable to configure project event notifications when managing group hooks through Terraform.

Closes #6789 (closed)

How to validate

The existing acceptance test TestAccGitlabGroupHook_basic was updated to include project_events = true in both full-config test steps. This test runs in the CI pipeline against a GitLab EE instance and verifies the full create/update/import lifecycle.

No feature flags are required.

MR acceptance checklist

• The change is backward compatible
• Tests added for the new attribute (updated TestAccGitlabGroupHook_basic)
• Documentation regenerated via make generate
• Follows the existing pattern used by all other event attributes (e.g., job_events)

Changes

• internal/provider/resource_gitlab_group_hook.go — added project_events to the struct, schema, create/update options, and state model
• internal/provider/resource_gitlab_group_hook_test.go — added project_events = true to both full-config acceptance test steps
• docs/resources/group_hook.md — regenerated docs