ci: update Kani GitHub Actions runners to ubuntu-24.04

GitHub is deprecating the ubuntu-20.04 runner, with removal scheduled for
2025-04-01 (see: https://github.com/actions/runner-images/issues/11101)
This commit updates workflow files to use ubuntu-24.04 instead.

.github/workflows/cron-daily-kani.yml

@@ -5,7 +5,7 @@ on:
     - cron: '59 23 * * *'       # midnight every day.
 jobs:
   run-kani:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
     steps:
       - name: 'Checkout your code.'
         uses: actions/checkout@v4

.github/workflows/rust.yml

@@ -296,7 +296,7 @@ jobs:
 
   Kani:
     name: Kani codegen - stable toolchain
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
     steps:
       - name: "Checkout repo"
         uses: actions/checkout@v4