docs(fargate): eksctl update command is deprecated, use upgrade instead

[skl]

Description

Whilst following this guide, I ran eksctl update cluster which appears to be deprecated:

2024-01-30 13:33:15 [!]  This command is to be deprecated. Please use 'eksctl upgrade cluster' instead

There seem to be many instances of eksctl update throughout the docs, I just changed the one instance that I tested.

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the userdocs directory)
• Manually tested
• Made sure the title of the PR is a good description that can go into the release notes
• (Core team) Added labels for change area (e.g. area/nodegroup) and kind (e.g. kind/improvement)

BONUS POINTS checklist: complete for good vibes and maybe prizes?! 🤯

• Backfilled missing tests for code in same general area 🎉
• Refactored something and made the world a better place 🌟

[github-actions]

Hello skl 👋 Thank you for opening a Pull Request in eksctl project. The team will review the Pull Request and aim to respond within 1-10 business days. Meanwhile, please read about the Contribution and Code of Conduct guidelines here. You can find out more information about eksctl on our website