[Linux CI] Upgrade to GCC 9 (#1522)

Author: josetr

.github/workflows/main.yml

@@ -9,7 +9,7 @@ jobs:
       fail-fast: false
       matrix:
         config:
-          - { os: ubuntu-20.04, platform: x64, cxx: g++-7, cc: gcc-7 }
+          - { os: ubuntu-20.04, platform: x64, cxx: g++-9, cc: gcc-9 }
           - { os: macos-10.15, platform: x64, cxx: clang++, cc: clang }
           - { os: windows-2019, platform: x86, vs: 2019 }
           - { os: windows-2019, platform: x64, vs: 2019 }