refactor: replace deprecated String.prototype.substr() (#1066)
refactor: replace deprecated String.prototype.substr() (#1066)

.substr() is deprecated so we replace it with .slice() which works similarily but isn't deprecated

Signed-off-by: Tobias Speicher <0bf48b1b307ef620053f01d201166f9ed9f81d78@gmail.com>

Co-authored-by: Daniel Lee <d192281538f777f863cb50d2c819c4df52cdb2f6@gmail.com>
firebase/firebase-functions,firebase/firebase-functions,firebase/firebase-functions