ci: revert to standard ubuntu-latest runners

- Change from ubuntu-latest-4-cores to ubuntu-latest runners
- 4-core runners may not be available/enabled for this repository
- Standard runners (2 cores, 7GB RAM) are always available and sufficient
- femu-compile.sh will still use make -j 2 for parallel builds

Author: huaicheng

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-latest-4-cores  # 4 cores, 16GB RAM for faster FEMU builds
+    runs-on: ubuntu-latest  # Standard runners: 2 cores, 7GB RAM
     strategy:
       matrix:
         os: [ubuntu-20.04, ubuntu-22.04, ubuntu-24.04]
@@ -86,7 +86,7 @@ jobs:
         retention-days: 7
 
   compatibility-test:
-    runs-on: ubuntu-latest-4-cores  # Match build job runner
+    runs-on: ubuntu-latest  # Standard runners
     needs: build
     if: success()