Update CI workflow to treat warnings as errors

The CI workflow configuration has been changed to treat warnings as errors in order to maintain a strict coding standard. We do this by adding `-D warnings` to the `cargo clippy` command. This change will help to prevent any future code with warnings from being merged.

.github/workflows/ci.yaml

@@ -18,5 +18,5 @@ jobs:
           curl -LsSf https://get.nexte.st/latest/linux | tar zxf - -C ${CARGO_HOME:-~/.cargo}/bin
       - run: cargo fmt --all --check
       - run: cargo check --workspace
-      - run: cargo clippy --all-targets --all-features
+      - run: cargo clippy --all-targets --all-features -- -D warnings
       - run: cargo nextest run -E 'all() - package(calendar)'