fix: use events.pagerduty.com for the US integration URL host

[sapslaj]

events.us.pagerduty.com doesn't seem to be a valid hostname for the events API endpoint. The UI and docs all point to events.pagerduty.com being the hostname. Even though events.us.pagerduty.com does seem to resolve, the SSL certificate is invalid.

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[sapslaj]

For anyone who stumbles across this PR with the same issue, there's some intrinsic function spaghetti you can do as a workaround:

!Sub
  - https://events.pagerduty.com/integration/${IntegrationKey}/enqueue
  - IntegrationKey: !Select
      - 4
      - !Split
          - /
          - !GetAtt PagerDutyServiceSubscription.IntegrationUrl

You can also just grab the !Select if you need just the integration key. In CDK, it's a little easier to grok (which is what I'm using instead of CloudFormation directly):

import { CfnResource, Fn, Token } from 'aws-cdk-lib';

// ...

const resource = new CfnResource(this, 'Resource', {
  type: 'PagerDuty::Services::Integration',
  properties: { /* ... */ },
});

const integrationKey = Fn.select(4, Fn.split('/', Token.asString(resource.getAtt('IntegrationUrl'))));
const integrationUrl = `https://events.pagerduty.com/integration/${integrationKey}/enqueue`;