ICU-22721 Add workflow that enforces required checks

[echeran]

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-_____
• Required: The PR title must be prefixed with a JIRA Issue number.
• Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• Required: Each commit message must be prefixed with a JIRA Issue number.
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

[echeran]

Oops, I thought I was opening this PR in my personal fork for testing purposes before I vetted and opened it up here in the upstream repo. I will test and if things seem okay, I'll open a PR again. Stay tuned...