Bump spdlog to 1.9.0

This is actually mostly exiting because it now supports fmtlib version
8, which is great! Sadly to fully utilize fmt 8.0.0, we need C++20
support. Check the [release
notes](https://github.com/fmtlib/fmt/releases/tag/8.0.0) of fmtlib 8.0.0
to see what awesome things we can do, if we switch to C++20 :^)
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