Modify labeler to consider issues with more than 3 services impacted as cross-provider

[roaks3]

Fixes hashicorp/terraform-provider-google#20818

• I tested locally with some issue bodies and the results looked good
• I chose not to include the backfill job: basically, the backfill job ignores issues with service/terraform, so the condition mentioned here would only be triggered after a ticket is created with < 4 services, and then more are added later, which is both unusual, and something we can handle manually.

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[modular-magician]

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

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