Bumps docker build action's ubuntu version (tgstation#89532)

## About The Pull Request

Github's nuking this runner next month.
Not tested, it's not as if this works rn anyway.

actions/runner-images#11101

.github/workflows/docker_publish.yml

@@ -6,7 +6,7 @@ on:
 jobs:
   publish:
     if: ( !contains(github.event.head_commit.message, '[ci skip]') )
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@v4