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.

[artonge]

Not quite the same, but in this case, it works :)

[nickvergessen]

@artonge feel free to merge

[nickvergessen]

cypress Expected — Waiting for status to be reported

This is a bit suspicious thou :P

[artonge]

@artonge feel free to merge

I can't, need more rights for that :'( (as CI is not fully green)

Edit: I can check why cypress is hanging

[CommanderRoot]

I have rebased the PR on the current master to hopefully restart the workflows and maybe make them work this time.

[nickvergessen]

Can you rebase on top of #789

[artonge]

Thanks for this @CommanderRoot !