fix kani build job

Ubuntu 20.04 has been retired.

Author: Freax13

.github/workflows/build.yml

@@ -176,7 +176,7 @@ jobs:
         run: cargo +stable semver-checks check-release
 
   kani:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
       - uses: Swatinem/rust-cache@v2