fix: rename locale `iso` property to `language`

[BobbieGoede]

🔗 Linked issue

• https://github.com/nuxt-modules/i18n/issues/2449

❓ Type of change

• [ ] 📖 Documentation (updates to the documentation, readme or JSdoc annotations)
• [ ] 🐞 Bug fix (a non-breaking change that fixes an issue)
• [ ] 👌 Enhancement (improving an existing functionality like performance)
• [ ] ✨ New feature (a non-breaking change that adds functionality)
• [x] 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• [ ] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

As discussed in https://github.com/nuxt-modules/i18n/issues/2449, this renames the iso key to language to better fit the usage of the property.

This PR should be backwards compatible but will log a warning when iso is being used instead of language. We should again remember to remove the backwards compatibility before releasing v9.

📝 Checklist

• [x] I have linked an issue or discussion.
• [ ] I have added tests (if possible).
• [x] I have updated the documentation accordingly.

[BobbieGoede]

@kazupon I made changes based on your feedback, let me know if anything needs to be changed!