build(deps): bump external/spdlog from 5ebfc92 to a3a0c9d

Bumps [external/spdlog](https://github.com/gabime/spdlog) from `5ebfc92` to `a3a0c9d`.
- [Release notes](https://github.com/gabime/spdlog/releases)
- [Commits](gabime/spdlog@5ebfc92...a3a0c9d)

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

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