Bump external/spdlog from ac55e60 to 1ef8d3c

Bumps [external/spdlog](https://github.com/gabime/spdlog) from `ac55e60` to `1ef8d3c`.
- [Release notes](https://github.com/gabime/spdlog/releases)
- [Commits](gabime/spdlog@ac55e60...1ef8d3c)

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

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