chore: bump MSRV to 1.91

Otherwise cargo install wasm-pack doesn't work

flake.nix

@@ -22,7 +22,7 @@
         };
 
         # MSRV from the project
-        rustVersion = "1.89.0";
+        rustVersion = "1.91.0";
         
         # Rust toolchain with required components
         rustToolchain = pkgs.rust-bin.stable.${rustVersion}.default.override {
@@ -159,4 +159,4 @@
           };
         };
       });
-}
+}