Bump external/fmt from 536cabd to df249d8 in the submodules group

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


Updates `external/fmt` from `536cabd` to `df249d8`
- [Release notes](https://github.com/fmtlib/fmt/releases)
- [Commits](fmtlib/fmt@536cabd...df249d8)

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

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