CI: Update unit tests from ubuntu-20.04 to ubuntu-24.04

GitHub retired their Ubuntu 20 image (for GitHub-hosted runners) on April 15, 2025. Therefore, we are forced to update.

https://github.blog/changelog/2025-01-15-github-actions-ubuntu-20-runner-image-brownout-dates-and-other-breaking-changes/

Author: DilumAluthge

.github/workflows/UnitTests.yml

@@ -208,7 +208,7 @@ jobs:
 
       fail-fast: false
 
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
 
     env:
       JULIA_MPI_TEST_BINARY: system
@@ -409,7 +409,7 @@ jobs:
 
       fail-fast: false
 
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
 
     env:
       JULIA_MPI_TEST_BINARY: MPItrampoline_jll