refactor: replace deprecated String.prototype.substr()

[CommanderRoot]

✨ Pull Request

📓 Referenced Issue

ℹ️ About the PR

String.prototype.substr() is deprecated so we replace it with String.prototype.slice() or String.prototype.substring() which work similarily but aren't deprecated. .substr() probably isn't going away anytime soon but the change is trivial so it doesn't hurt to do it.

🖼️ Testing Scenarios / Screenshots

[CLAassistant]

All committers have signed the CLA.