CI: Use correct MSRV toolchain

In commit 2a40a35 (PR rust-bitcoin#3418) I copied the `rust.yml` file from
`master` and in doing so used the wrong MSRV toolchain.

Use the correct MSRV toolchain for this branch.

Author: tcharding

.github/workflows/rust.yml

@@ -88,7 +88,7 @@ jobs:
       - name: "Select toolchain"
         uses: dtolnay/rust-toolchain@stable
         with:
-          toolchain: "1.63.0"
+          toolchain: "1.56.1"
       - name: "Set dependencies"
         run: cp Cargo-${{ matrix.dep }}.lock Cargo.lock
       - name: "Run test script"