chore(ci): update to ubuntu 24.04 (#1045)

GitHub Actions dropped 20.04 image: actions/runner-images#11101

Author: Rushaway

.github/workflows/plugin.yml

@@ -25,9 +25,9 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-20.04]
+        os: [ubuntu-24.04]
         include:
-          - os: ubuntu-20.04
+          - os: ubuntu-24.04
     steps:
       - uses: actions/checkout@v4
       - name: Build sourcemod plugin