feat: update plugin @netlify/plugin-nextjs to version 5.9.1

[pieh]

Thanks for contributing the Netlify plugins directory!

Are you adding a plugin or updating one?

• Adding a plugin
• Updating a plugin

Have you completed the following?

• Read and followed the plugin author guidelines.
• Included all required fields in your entry.
• Tested the plugin locally and on Netlify, using the plugin version stated in your entry.

Test plan
Please add a link to a successful public deploy log using the stated version of the plugin. Include any other context reviewers might need for testing.

[netlify]

✅ Deploy Preview for netlify-plugins ready!

Name	Link
🔨 Latest commit	5849e34
🔍 Latest deploy log	https://app.netlify.com/sites/netlify-plugins/deploys/6762d89f3ccd5e0008af4191
😎 Deploy Preview	https://deploy-preview-1381--netlify-plugins.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.