fmt: pickup v9.1.0

fmt 9.0.0 dropped automatic `std::ostream` insertion operator discovery
when `fmt/ostream.h` to prevent ODR violations. instead of defining
`FMT_DEPRECATED_OSTREAM`, we took efforts to specialize
`fmt::formatter<..>` to be compatible with the new fmtlib. to avoid
breaking the build with fmt v9 and up, let's bump up the fmt submodule.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>

diff --git a/src/fmt b/src/fmt
index b6f4ceaed0a0a24ccf575fab6c56dd50ccf6f1a9..a33701196adfad74917046096bf5a2aa0ab0bb50 160000 (submodule)
--- a/src/fmt
+++ b/src/fmt
@@ -1 +1 @@
-Subproject commit b6f4ceaed0a0a24ccf575fab6c56dd50ccf6f1a9
+Subproject commit a33701196adfad74917046096bf5a2aa0ab0bb50