Fix unknown before apply warnings for namespace endpoints

[bechols]

Currently, when updating retention on a namespace, the namespace endpoints are marked "known after apply":

# temporalcloud_namespace.foo-terraform will be updated in-place
  ~ resource "temporalcloud_namespace" "foo-terraform" {
      ~ endpoints      = {
          ~ grpc_address      = "us-east-1.aws.api.temporal.io:7233" -> (known after apply)
          + mtls_grpc_address = (known after apply)
          ~ web_address       = "https://foo-terraform.abc12.web.tmprl.cloud/" -> (known after apply)
        } -> (known after apply)
        id             = "foo-terraform.abc12"
        name           = "foo-terraform"
      ~ retention_days = 1 -> 2
        # (2 unchanged attributes hidden)
    }

Plan: 0 to add, 1 to change, 0 to destroy.

Do you want to perform these actions?

This creates concern that the endpoints could change and thus break clients. These endpoints aren't actually editable, but the current experience is confusing/worrying.

We believe this will fix it:

	PlanModifiers: []planmodifier.String{
					stringplanmodifier.UseStateForUnknown(),
				},