gha: ubuntu-20.04 -> ubuntu-latest

ubuntu-20.04 was deprecated and removed from use on 20250415. Updating
to latest prevents us from having this issue in the future. As this is a
pretty low-consequence test, I feel comfortable using the latest version
here.

Signed-off-by: Ethan Carter Edwards <[email protected]>

Author: ethancedwards8

.github/workflows/check-syntax.yml

@@ -4,7 +4,7 @@ on: [push, pull_request]
 
 jobs:
   test:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
     - name: checkout
       uses: actions/checkout@master