make subnetwork region forcenew and update its description

[danawillow]

Fixes hashicorp/terraform-provider-google#6470

Release Note Template for Downstream PRs (will be copied)

compute: fixed issue where trying to update the region of `google_compute_subnetwork` would fail instead of destroying/recreating the subnetwork

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 3 files changed, 4 insertions(+), 3 deletions(-))
Terraform Beta: Diff ( 3 files changed, 4 insertions(+), 3 deletions(-))
Ansible: Diff ( 2 files changed, 4 insertions(+), 4 deletions(-))
Inspec: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))