Update GitHub CI run image

GitHub runs include this warning

    The Ubuntu-20.04 brownout takes place from 2025-02-01.
    For more details, see actions/runner-images#11101

Let's try not to depend on particular image for these jobs. The exact
image *usually* does not matter and we don't pin image for all the other
jobs.

.github/workflows/rust.yml

@@ -41,7 +41,7 @@ on:
 jobs:
   # Check license header
   license-header-check:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     name: Check License Header
     steps:
       - uses: actions/checkout@v4
@@ -389,7 +389,7 @@ jobs:
   test-datafusion-pyarrow:
     name: cargo test pyarrow (amd64)
     needs: linux-build-lib
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     container:
       image: amd64/rust:bullseye # Workaround https://github.com/actions/setup-python/issues/721
     steps: