Bump external/fmt from 873670b to 7c3d015 in the submodules group

Bumps the submodules group with 1 update: [external/fmt](https://github.com/fmtlib/fmt).


Updates `external/fmt` from `873670b` to `7c3d015`
- [Release notes](https://github.com/fmtlib/fmt/releases)
- [Commits](fmtlib/fmt@873670b...7c3d015)

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

Signed-off-by: dependabot[bot] <[email protected]>