Current resource

resource "mongodbatlas_cluster" "cluster" {
  project_id   = var.project_id
  name         = var.cluster_name
  disk_size_gb = var.disk_size_gb
  num_shards   = var.num_shards
  replication_factor           = var.replication_factor
  auto_scaling_disk_gb_enabled = var.auto_scaling_disk_gb_enabled
  mongo_db_major_version       = var.mongo_db_major_version
  //Provider Settings "block"
  provider_name               = var.provider_name
  provider_disk_iops          = var.provider_disk_iops
  provider_volume_type        = var.provider_volume_type
  provider_instance_size_name = var.provider_instance_size_name
  provider_region_name        = var.provider_region_name
  provider_backup_enabled      = var.provider_backup_enabled
}

Suggested resource

resource "mongodbatlas_cluster" "cluster" {
  project_id   = var.project_id
  name         = var.cluster_name
  disk_size_gb = var.disk_size_gb
  num_shards   = var.num_shards
  replication_factor           = var.replication_factor
  auto_scaling_disk_gb_enabled = var.auto_scaling_disk_gb_enabled
  mongo_db_major_version       = var.mongo_db_major_version
  //Provider Settings "block"
  cluster_provider = {
    name               = var.provider_name
    disk_iops          = var.provider_disk_iops
    volume_type        = var.provider_volume_type
    instance_size_name = var.provider_instance_size_name
    region_name        = var.provider_region_name
    backup_enabled     = var.provider_backup_enabled
  }
}

cluster_provider property will be a map of configuration for provider.