refactor: replace deprecated String.prototype.substr()

[CommanderRoot]

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

[linux-foundation-easycla[bot]]

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• :white_check_mark: login: CommanderRoot (84fa7cb521d298d696fcd3fa70b9a9b5187dadad)

[acao]

@CommanderRoot can you sign the CLA please?

[CommanderRoot]

I apologize for not signing the CLA sooner, must have somehow missed it