refactor: replace deprecated .substr() with .substring()

[khatri2002]

The .substr() method has been replaced with .substring() as .substr() is deprecated.

Reference: Mozilla Developer Documentation

This update ensures better compatibility with modern web standards and prevents potential issues in future browser updates.

[jitsi-jenkins]

Hi, thanks for your contribution!
If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.

[khatri2002]

Hi, thanks for your contribution! If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.

Thanks for the update! I have signed the Individual Contributor License Agreement