Bump external/fmt from 720da57 to 536cabd in the submodules group (…

…#256)

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


Updates `external/fmt` from `720da57` to `536cabd`
- [Release notes](https://github.com/fmtlib/fmt/releases)
- [Commits](fmtlib/fmt@720da57...536cabd)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>