Update GitHub workflows

[StephanTLavavej]

• Delete GitHub workflows for the old Code Reviews project.
  • "Assign new PRs" is built-in functionality for new-style projects - except for the draft distinction, which we can live without.
  • The /pr wip and /pr review commands were rarely used by contributors, and now that the backlog is much smaller, they aren't worth replacing/maintaining. (I believe they broke when MS started requiring permissions and I didn't grant them in Add permissions to the Assign New PR action #4223.)
• update-status-chart.yml: .ts => .mts
  • I'll need to merge Status Chart: Modernize modules, chart.js 4.4.2 #4460 on the same day as this.
• Update node-version to 21.7.1.
  • Updating this version is always "nice to have" - the infrastructure usually retrieves the latest (and is already retrieving 21.7.1). I just don't want to let it get too outdated, in case some stale version starts getting selected from a cache, significantly diverging from what I'm locally using.

⚠️ Note to self:

After merging, we should update the repo's settings for Actions Permissions.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.