Use 20.04 for gcc-9 and above

.github/workflows/ci.yml

@@ -45,13 +45,13 @@ jobs:
             install: g++-8
           - toolset: gcc-9
             cxxstd: "03,11,14,17,2a"
-            os: ubuntu-18.04
+            os: ubuntu-20.04
           - toolset: gcc-10
             cxxstd: "03,11,14,17,2a"
-            os: ubuntu-18.04
+            os: ubuntu-20.04
           - toolset: gcc-11
             cxxstd: "03,11,14,17,2a"
-            os: ubuntu-18.04
+            os: ubuntu-20.04
             install: g++-11
           - toolset: clang
             compiler: clang++-3.5