ci: switch Bitcoin Core checkout back to master

Author: Shourya742

.github/workflows/ci.yml

@@ -25,7 +25,7 @@ jobs:
           key: ccache-${{ runner.os }}-${{ github.sha }}
           restore-keys: ccache-${{ runner.os }}-
       - name: Checkout Bitcoin Core
-        run: git clone --depth 1 --branch 31.x https://github.com/bitcoin/bitcoin.git
+        run: git clone --depth 1 --branch master https://github.com/bitcoin/bitcoin.git
       - name: Build Bitcoin Core
         run: |
           cd bitcoin