bump oldest supported CMake in the CI to 3.15

Author: matekelemen

.github/workflows/ci.yml

@@ -381,14 +381,14 @@ jobs:
         python3.8 run_python_tests.py
 
 
-  Ubuntu-old-CMake:
+  Ubuntu-minimum-CMake:
     runs-on: ubuntu-latest
 
     steps:
     - name: Setup cmake
-      uses: jwlawson/actions-setup-cmake@v1.10
+      uses: jwlawson/actions-setup-cmake@v2
       with:
-        cmake-version: '2.8.x'
+        cmake-version: '3.15.x'
 
     - name: Use cmake
       run: cmake --version