SWP: Add support for value "EXPLICIT_ROUTING_MODE" on field "routing_mode" on resource "google_network_services_gateway"

[Samir-Cit]

Hello folks.

This PR is to fix hashicorp/terraform-provider-google#19826

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gateway: added `EXPLICIT_ROUTING_MODE` value on `routing_mode` field on `google_network_services_gateway` resource

[github-actions]

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[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

google provider: Diff ( 2 files changed, 8 insertions(+), 6 deletions(-))
google-beta provider: Diff ( 2 files changed, 8 insertions(+), 6 deletions(-))
terraform-google-conversion: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))

[Samir-Cit]

I'll push a new version of this code ASAP

[modular-magician]

Tests analytics

Total tests: 60
Passed tests: 58
Skipped tests: 1
Affected tests: 1

Click here to see the affected service packages

• networkservices

#### Action taken

Found 1 affected test(s) by replaying old test recordings. Starting RECORDING based on the most recent commit. Click here to see the affected tests

• TestAccNetworkServicesGateway_update

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[modular-magician]

🔴 Tests failed during RECORDING mode:
TestAccNetworkServicesGateway_update [Error message] [Debug log]

🔴 Errors occurred during RECORDING mode. Please fix them to complete your PR.

View the build log or the debug log for each test