Update CI image

Author: o01eg

.github/workflows/ci.yml

@@ -40,10 +40,10 @@ jobs:
 
   linux-cmake:
     name: Build (Linux, GCC, CMake)
-    runs-on: ubuntu-16.04
+    runs-on: ubuntu-18.04
     steps:
       - name: Checkout
-        uses: actions/checkout@v2.3.4
+        uses: actions/checkout@v2
         with:
           submodules: recursive
 
@@ -70,10 +70,10 @@ jobs:
 
   linux-cmake-ninja:
     name: Build (Linux, GCC, CMake Ninja)
-    runs-on: ubuntu-16.04
+    runs-on: ubuntu-18.04
     steps:
       - name: Checkout
-        uses: actions/checkout@v2.3.4
+        uses: actions/checkout@v2
         with:
           submodules: recursive