Upgrade the version of rustc

I missed this in commit 5c5319 - I misinterpreted the benchmark failure
as a regression that I'd acknowledged.

Author: seddonym

.github/workflows/main.yml

@@ -74,7 +74,7 @@ jobs:
             python-version: 3.14
 
         - name: Update to a Cargo version that supports 2024 edition
-          run: rustup install 1.90.0 && rustup default 1.90.0
+          run: rustup install 1.96.1 && rustup default 1.96.1
 
         - name: Run benchmarks
           uses: CodSpeedHQ/action@v4