build(eslint): upgrade to v9

[matthewgallo]

Closes #6187

These changes upgrade eslint to v9 and moves our config from .eslintrc.js to eslint.config.mjs, using the new flat config. I picked up where @elycheea left off with this one, so shout out to Elysia 🎉

What did you change?

eslint config related files
Removed some disabling of rules

How did you test and verify your work?

Manually ran npx eslint from project root

[netlify]

✅ Deploy Preview for ibm-products-web-components ready!

Name	Link
🔨 Latest commit	5c1cbc5
🔍 Latest deploy log	https://app.netlify.com/sites/ibm-products-web-components/deploys/679cee3728f11e0008ee8479
😎 Deploy Preview	https://deploy-preview-6825--ibm-products-web-components.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[netlify]

✅ Deploy Preview for carbon-for-ibm-products ready!

Name	Link
🔨 Latest commit	5c1cbc5
🔍 Latest deploy log	https://app.netlify.com/sites/carbon-for-ibm-products/deploys/679cee37d1a2710008c9d8b6
😎 Deploy Preview	https://deploy-preview-6825--carbon-for-ibm-products.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.25%. Comparing base (d043ffa) to head (5c1cbc5).
Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6825      +/-   ##
==========================================
+ Coverage   81.17%   81.25%   +0.08%     
==========================================
  Files         398      397       -1     
  Lines       12956    12949       -7     
  Branches     4283     4281       -2     
==========================================
+ Hits        10517    10522       +5     
+ Misses       2439     2427      -12     

Components	Coverage Δ
ibm-products	∅ <ø> (∅)
ibm-products-web-components	∅ <ø> (∅)

[elycheea]

I only did the easy part of the update to find the initial blockers 🙈. Thanks for finishing it out!

We also have good timing since System released yesterday so we can update eslint-config-carbon to the full release instead of the RC. 🎉

[davidmenendez]

🚀