ci: bump fuzz MSRV to 1.65

This is just copied from rust-bitcoin

Author: apoelstra

.github/workflows/fuzz.yml

@@ -40,7 +40,7 @@ roundtrip_confidential,
           key: cache-${{ matrix.target }}-${{ hashFiles('**/Cargo.toml','**/Cargo.lock') }}
       - uses: actions-rs/toolchain@v1
         with:
-          toolchain: 1.58
+          toolchain: 1.65
           override: true
           profile: minimal
       - name: fuzz