chore(ci): Lock docs to MSRV

This will help avoid issues with new warnings

Author: epage

.github/workflows/ci.yml

@@ -137,7 +137,7 @@ jobs:
     - name: Install Rust
       uses: actions-rs/toolchain@v1
       with:
-        toolchain: stable
+        toolchain: 1.60.0  # MSRV
         profile: minimal
         override: true
     - uses: Swatinem/rust-cache@v1