fix: merge values for synthetic monitor locations

[biscout42]

due-to elastic/kibana#194901

Tested local with Elastic cloud via copying binaries to

~/.terraform.d/plugins/local/elastic/elasticstack/0.11.8/darwin_arm64/terraform-provider-elasticstack

and using

terraform {
  required_version = ">= 1.0.0"

  required_providers {
    ec = {
      source = "elastic/ec"
    }
    elasticstack = {
      source  = "local/elastic/elasticstack"
      version = "0.11.8"
    }
  }
}