Raise minimum tested compiler to rust 1.67

Required by dev-dependency on flate2.

    error: package `flate2 v1.1.0` cannot be built because it requires
    rustc 1.67.0 or newer, while the currently active rustc version is 1.63.0
    Either upgrade to rustc 1.67.0 or newer, or use
    cargo update -p [email protected] --precise ver
    where `ver` is the latest version of `flate2` supporting rustc 1.63.0

Author: dtolnay

.github/workflows/ci.yml

@@ -24,7 +24,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [1.63.0, stable, beta]
+        rust: [1.67.0, stable, beta]
     timeout-minutes: 45
     steps:
       - uses: actions/checkout@v4