Update Iosevka font to 15.3.1
**Requirements**
- [X] I have searched the [issues](https://github.com/ryanoasis/nerd-fonts/issues) for my request and found nothing related and/or helpful
- [X] I have searched the [FAQ](https://github.com/ryanoasis/nerd-fonts/wiki/FAQ-and-Troubleshooting) for help
- [X] I have searched the [Wiki](https://github.com/ryanoasis/nerd-fonts/wiki) for help

**Is your feature request related to a problem? Please describe.**
The current version of the Iosevka fonts is 15.3.1. It includes many new glyphs that aren't available in 11.0.1, as well as changes to existing glyphs, most notably more readable 0, 4, 6 and 9 digits.

**Describe the solution you'd like**
The font in this repo should ideally be updated to feature the version 15.3.1, with any glyphs missing from the Nerd Fonts set (Iosevka already supports Powerline, for example) patched in.

**Describe alternatives you've considered**
I don't think that this section applies here.