compute: added network_id to google_compute_network

[modular-magician]

Add network_id to google_compute_network.

It's an integer, not a string, and follows the expected convention for naming.

This adds a note deprecating numeric_id (to be potentially removed at some later date), which will have the same value, but as a string.

Part of terraform-provider-google#20530

See also #12351 and #12339, in particular this comment, this comment, and this discussion

Let me know if you want me to break this up into two separate release notes.

Release Note Template for Downstream PRs (will be copied)

See Write release notes for guidance.

compute: added `network_id` (integer) to `google_compute_network` resource and data source

compute: deprecated `numeric_id` (string) field in `google_compute_network` resource. Use the new `network_id` (integer)  field instead 

Derived from GoogleCloudPlatform/magic-modules#12504