BREAKING CHANGE: upgrade to Lit 3

[dlockhart]

⚠️ DO NOT MERGE yet! ⚠️

👋 Hey, we're upgrading to Lit 3! 🎉 This PR prepares this repo for the upgrade, which will happen starting May 21st. Someone from team Gaudí will take care of everything (except approving this PR), so sit back and enjoy the upgrade!

A Phased Approach

Merges will happen in several phases based on where each repo sits in the overall dependency graph of Lit-related things.

This repo will be in phase: 1 of 10

Since this repo is in the first phase, it doesn't have to wait for any dependencies to perform a major release, so its CI should be green immediately.

Next steps

• [ ] CODEOWNERS approvals received 👍 (you can do that now to avoid holding up the upgrade later!)
• [ ] Wait until this repo's phase of the upgrade arrives ⌛
• [ ] Rebase from main / master to pick up latest changes
• [ ] If this repo has a package-lock.json, update to latest dependencies
• [ ] CI goes green ✅
• [ ] Merge & trigger a MAJOR version bump

[dlockhart]

Experimenting with the PR description on us -- let me know if you can think of things to change before I repeat it 100+ times!

[github-actions[bot]]

Thanks for the PR! 🎉

We've deployed an automatic preview for this PR - you can see your changes here:

URL	https://live.d2l.dev/prs/BrightspaceUI/core/pr-4644/

[!NOTE] The build needs to finish before your changes are deployed. Changes to the PR will automatically update the instance.

[d2l-github-release-tokens[bot]]

:tada: This PR is included in version 3.0.0 :tada:

The release is available on:

• GitHub release
• npm package (@latest dist-tag)

Your semantic-release bot :package::rocket: