Remove ubuntu-20.04 from github actions

This is fully unsupported [1], and prevents further tests from running.

[1]: https://github.com/actions/runner-images/issues/11101
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 57730c0..98ff15c 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -41,7 +41,9 @@
   compiler:
     strategy:
       matrix:
-        os: [ubuntu-20.04, ubuntu-22.04, ubuntu-24.04]
+        os:
+          - ubuntu-22.04
+          - ubuntu-24.04
       fail-fast: false
     runs-on: ${{ matrix.os }}
     steps: