ci: replace ubuntu-20.04 workflows with newer ones

GitHub Actions has deprecated `ubuntu-20.04`, its jobs are getting
canceled often and it is going to be completely removed in a few weeks,
so let's replace it with the newer `ubuntu-24.04` version.

Author: drewdzzz

.github/workflows/testing.yml

@@ -14,8 +14,8 @@ jobs:
       fail-fast: false
       matrix:
         runs-on:
-          - ubuntu-20.04
           - ubuntu-22.04
+          - ubuntu-24.04
           - macos-13
           - macos-14
           - macos-15
@@ -35,8 +35,8 @@ jobs:
       fail-fast: false
       matrix:
         runs-on:
-          - ubuntu-20.04
           - ubuntu-22.04
+          - ubuntu-24.04
           - macos-13
           - macos-14
           - macos-15