CI: Update unit tests from ubuntu-20.04 to ubuntu-24.04 (#899)

* 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/

* [CI] Currently mpich from Ubuntu repo, it's broken

---------

Co-authored-by: Mosè Giordano <m.giordano@ucl.ac.uk>

Author: DilumAluthge

.github/workflows/UnitTests.yml

@@ -201,14 +201,15 @@ jobs:
     strategy:
       matrix:
         mpi:
-          - libmpich-dev
+          # MPICH in Ubuntu is currently broken: https://github.com/pmodels/mpich/issues/7064
+          # - libmpich-dev
           - libopenmpi-dev
         julia_version:
           - "1"
 
       fail-fast: false
 
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
 
     env:
       JULIA_MPI_TEST_BINARY: system
@@ -397,7 +398,8 @@ jobs:
     strategy:
       matrix:
         mpi:
-          - libmpich-dev
+          # MPICH in Ubuntu is currently broken: https://github.com/pmodels/mpich/issues/7064
+          # - libmpich-dev
           - libopenmpi-dev
         julia_version:
           - "1"
@@ -409,7 +411,7 @@ jobs:
 
       fail-fast: false
 
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
 
     env:
       JULIA_MPI_TEST_BINARY: MPItrampoline_jll