docs: Change timezone picker to modern, working alternative

[ItsNiceCraft]

Description

This PR changes the comment in the _config.yml file to link to a properly working, modern timezone picker

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (refactoring and improving code)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Additional context

How has this been tested

• I have run bash ./tools/test (at the root of the project) locally and passed
• I have tested this feature in the browser

[ItsNiceCraft]

Context: the current timezone picker is old and doesn't currently work (403 Forbidden when trying to submit)

[cotes2020]

Maybe http://www.timezoneconverter.com/cgi-bin/findzone/findzone isn't accessible in the region where you live, but it's working.

Even so, the site you recommend looks like a better alternative.