Extension creator upgrade paths - Update documentation

[jordojordo]

Summary

Fixes #12035
Fixes #12036

This will re-implement the update script for extensions using the --update option. It allows extension developers to upgrade their extensions from legacy-v1, to legacy-v2.

Occurred changes and/or fixed issues

• Switched execSync calls in the main creator script to spawnSync to pass arguments as an array
• Fixed an issue with the upgrade.sh script not being called due to a typo
• Updated documentation to include information on how to build/create an extension for different Rancher versions, as well as how to upgrade to be compatible with a newer version of Rancher

Checklist

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[aalves08]

LGTM mate!