Fixing fmt version to compile spdlog

According to gabime/spdlog#1975 spdlog will not compile with fmt 8.0.0, that is the version cached in Github Actions Runners. If I confirm that I'll not use spdlog, I can reverse this change. Right now I just want to see if the CI will run all the way.

conanfile.txt

@@ -1,5 +1,5 @@
 [requires]
-fmt/[>=7.1.3]
+fmt/[=7.1.3]
 spdlog/[>=1.8.5]
 catch2/[>=2.13.6]
 ctre/[>=3.4.1]