Update MSRV to 1.67

There is surely a better way of doing this, but this is low barrier to entry.

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: ["1.66", stable]
+        rust: ["1.67", stable]
     steps:
     - uses: actions/checkout@v3
     - uses: dtolnay/rust-toolchain@master