Update maven-pr-builder.yml

upgraded build server runtime from Ubuntu 18.04 to 20.04 LTS for the PR builder as well

Author: ohbus

.github/workflows/maven-pr-builder.yml

@@ -33,7 +33,7 @@ on:
 jobs:
   build:
 
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v2