Drop MSRV to 1.63

Debian bookworm ships with 1.63, and since the change is trivial,
there's little reason to not support people running mempool on
Debian.

.github/workflows/ci.yml

@@ -27,8 +27,8 @@ jobs:
           node-version: ${{ matrix.node }}
           registry-url: "https://registry.npmjs.org"
 
-      - name: Install 1.70.x Rust toolchain
-        uses: dtolnay/rust-toolchain@1.70
+      - name: Install 1.63.x Rust toolchain
+        uses: dtolnay/rust-toolchain@1.63
 
       - name: Install
         if: ${{ matrix.flavor == 'dev'}}