Adopt Pagerslack for dev oncall officially

Why is this change being made?

Development representatives reviewed the First Responder pilot program and decided to adopt Pagerslack for the dev oncall program #8964 (comment 494348526). Meanwhile, taking the opportunity to polish up the process documentation.

Review app link: https://cdu-dev-oncall-formal-pagerslack-adoption.about.gitlab-review.app/handbook/engineering/development/processes/Infra-Dev-Escalation/process.html

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
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