Increase msrv to 1.81.0

Author: martynp

.github/workflows/actions.yml

@@ -37,9 +37,9 @@ jobs:
     steps:
 
     - uses: actions/checkout@v4
-    - uses: dtolnay/rust-toolchain@1.73.0
+    - uses: dtolnay/rust-toolchain@1.81.0
 
     - name: Build & Test
       run: |
         cargo build --release
-        cargo test
+        cargo test