Discard CI coverage of backtrace feature on toolchains older than 1.63

The backtrace crate's `cc` dependency now requires 1.63+.

Author: dtolnay

.github/workflows/ci.yml

@@ -50,7 +50,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [1.65.0, 1.59.0, 1.55.0, 1.52.0, 1.51.0, 1.50.0, 1.39.0]
+        rust: [1.65.0, 1.52.0, 1.51.0, 1.50.0, 1.39.0]
     timeout-minutes: 45
     steps:
       - uses: actions/checkout@v4