Swap all use of FontAwesome to the v5+ syntax and set all icons to be fa-solid. (#887)

### Prerequisites

Put an `x` into the box(es) that apply:

- [ ] This pull request fixes a bug.
- [ ] This pull request adds a feature.
- [ ] This pull request introduces breaking change.

### Description

This change is the second half of #882, designed to remove use of the v4
syntax from the codebase.

This is not removing the v4 shims as users may require it for their own
content, but we don't need the template to rely on the old syntax!

PLEASE NOTE: There are minor visible changes here by explicitly setting
a consistent font choice. The default fa-solid calendar icon has no
little squares where the days should be (but fa-calendar-days does, so
we could always swap back) and the default clock is solid (forkawesome
only had a hollow clock called clock-o).


![before-after](https://github.com/luizdepra/hugo-coder/assets/3343403/a2f95d34-3d7a-49e9-92c2-26f685bff601)

### Issues Resolved

I promised this cleanup in #858 

### Checklist

Put an `x` into the box(es) that apply:

#### General

- [x] Describe what changes are being made
- [x] Explain why and how the changes were necessary and implemented
respectively
- [x] Reference issue with `#<ISSUE_NO>` if applicable

#### Resources

- [ ] If you have changed any SCSS code, run `make release` to
regenerate all CSS files

#### Contributors

- [x] Add yourself to `CONTRIBUTORS.md` if you aren't on it already

Co-authored-by: Luiz F. A. de Prá <luizdepra@users.noreply.github.com>