Switch from Gerald to use native CODEOWNERS file (#1968)

## Summary:

I would like to start using Dependabot to help keep some deps up to date. We already have Dependabot configured to update two Khan Academy lint packages, but currently the Gerald check fails because it needs access to an org-wide secret (which it doesn't have access to). The good news is that this repo doesn't really use Gerald very much (only two rules). These rules are easily re-created with the [`CODEOWNERS`](https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners#example-of-a-codeowners-file) file instead. 

Issue: "none"

## Test plan:

We'll need to land this PR and watch the next few PRs and see if it tags the team correctly. The failure mode is just that we won't be tagged as a team.

Author: jeremywiebe

Reviewers: SonicScrewdriver

Required Reviewers:

Approved By: SonicScrewdriver

Checks: ✅ Publish npm snapshot (ubuntu-latest, 20.x), ✅ Cypress (ubuntu-latest, 20.x), ✅ Lint, Typecheck, Format, and Test (ubuntu-latest, 20.x), ✅ Check for .changeset entries for all changed files (ubuntu-latest, 20.x), ✅ Publish Storybook to Chromatic (ubuntu-latest, 20.x), ✅ Check builds for changes in size (ubuntu-latest, 20.x)

Pull Request URL: #1968

.github/CODEOWNERS

@@ -0,0 +1,5 @@
+# Khan/perseus is tagged for all PRs
+@Khan/perseus
+
+# AI Utils are owned by Tutor Platform
+widget-ai-utils/* @Khan/tut