Breaking changes: removal of various deprecated fields/resources in 18.0

Removal of the following:

• resource_gitlab_group_cluster.go and related files. Deprecated in 14.5
• resource_gitlab_instance_cluster.go and related files. Deprecated in 14.5
• resource_gitlab_project_cluster.go and related files. Deprecated in 14.5
• resource_gitlab_topic.go, soft_destroy functionality. Deprecated in 14.9.
• resource_gitlab_project.go, squash_option version logic. Added in 14.1.
• schema_gitlab_application_settings.go, remove hashed_storage_enabled field. Deprecated in 13.0.
• schema_gitlab_application_settings.go, remove repository_storages field. Deprecated in 13.0.
• schema_gitlab_project_variable.go, remove ForceNew: true from environment_scope field. Update ability added in 13.4.
• resource_gitlab_project.go, remove build_coverage_regex field. Deprecated in 15.0.
• resource_gitlab_topic.go, change title field to Required: true and logic manually enforcing required nature. Required from 15.0.
• resource_gitlab_integration_emails_on_push.go, remove gitlab_service_emails_on_push resource registration. Deprecated in 16.0.
• resource_gitlab_group.go, remove default_branch_protection field. TBC but potentially removed in 18.0: gitlab#408315
• resource_gitlab_integration_jira.go, remove project_key field. Deprecated in 17.0.
• resource_gitlab_project_approval_rule.go, remove 17.2 logic around report_type field validation.
• resource_gitlab_project.go, remove 17.3 logic around secret push protection.
• resource_gitlab_member_role.go, remove 17.3 logic.