Update Rust version

Something in 1.70+ is causing an invalid pointer dereference with
mmap-bitvec, which needs to be investigated and fixed.

Author: boydgreenfield

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
       - uses: actions-rs/toolchain@v1
         with:
           profile: minimal
-          toolchain: 1.60.0
+          toolchain: 1.69.0
           override: true
 
       - name: version info