build(deps): Bump external/fmt from 873670b to d8a79ea

Bumps [external/fmt](https://github.com/fmtlib/fmt) from `873670b` to `d8a79ea`.
- [Release notes](https://github.com/fmtlib/fmt/releases)
- [Commits](fmtlib/fmt@873670b...d8a79ea)

---
updated-dependencies:
- dependency-name: external/fmt
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>