chore: [M3-9606] - Use unstable_createBreakpoints to define our MUI breakpoints

[bnussman-akamai]

Description 📝

• Migrates from a hacky way of creating breakpoints to what seems like a slightly less hacky way
  • The function is prefixed with unstable_, but I think it's okay. Hopefully MUI will promote it to stable soon. Worst case we just revert this PR if we start seeing issues
  • The createBreakpints function has a confusing history. It existed a long time ago and we used it, but then it was removed in v5 (which is when we introduced the hacky approach), but then it was added back as unstable_createBreakpoints recently. Hopefully it is here to stay.... 🤞
  • In most cases, you can just define the breakpoints in the theme itself, but for our codebase, it is not that straightforward. See refactor: [M3-9591] - Cleanup token organization and paths #11867 (comment) for a long explanation

How to test 🧪

• Checkout the PR or view preview link
• Verify breakpoints still work as they did before as you shrink/expand the viewport

Author Checklists

As an Author, to speed up the review process, I considered 🤔

👀 Doing a self review
❔ Our contribution guidelines
🤏 Splitting feature into small PRs
➕ Adding a changeset
🧪 Providing/improving test coverage
🔐 Removing all sensitive information from the code and PR description
🚩 Using a feature flag to protect the release
👣 Providing comprehensive reproduction steps
📑 Providing or updating our documentation
🕛 Scheduling a pair reviewing session
📱 Providing mobile support
♿ Providing accessibility support

• I have read and considered all applicable items listed above.

As an Author, before moving this PR from Draft to Open, I confirmed ✅

• All unit tests are passing
• TypeScript compilation succeeded without errors
• Code passes all linting rules

[linode-gh-bot]

Cloud Manager UI test results

🎉 593 passing tests on test run #2 ↗︎

❌ Failing	✅ Passing	↪️ Skipped	🕐 Duration
0 Failing	593 Passing	4 Skipped	102m 47s

[hana-akamai]

Didn't notice any breakpoint regressions, thanks Banks! 🧹

[bill-akamai]

Breakpoints look good - thanks!