Add default value for advertise_mode on compute_router_peer

[sebglon]

Release Note Template for Downstream PRs (will be copied)

compute: fixed issue where the default value for the attribute `advertise_mode` on `google_compte_router_peer` was not populated on import

Fixes hashicorp/terraform-provider-google#6259

[modular-magician]

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[modular-magician]

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

Diff report:

Terraform GA: Diff ( 1 file changed, 4 insertions(+))
Terraform Beta: Diff ( 1 file changed, 4 insertions(+))

[danawillow]

LGTM, thanks @sebglon!

FYI I updated your PR description to make the changelog note a bit more clear and also to auto-close the bug that this fixes- take a look if you're curious.