message: Update deprecated Linux environment
  This changelist updates a deprecated Linux environment in our build matrix for GitHub Actions, switching from ubuntu-18.04 to ubuntu-20.04.
diff: @@ -18,17 +18,10 @@ jobs:
  fail-fast: false
  matrix:
  include:
  - - name: Linux_GCC_6_Python27
  - os: ubuntu-18.04
  - compiler: gcc
  - compiler_version: "6"
  - python: 2.7
  - cmake_config: -DMATERIALX_PYTHON_VERSION=2 -DMATERIALX_BUILD_VIEWER=OFF
  -
  - - name: Linux_GCC_7_Python37
  - os: ubuntu-18.04
  + - name: Linux_GCC_9_Python37
  + os: ubuntu-20.04
  compiler: gcc
  - compiler_version: "7"
  + compiler_version: "9"
  python: 3.7
  cmake_config: -DMATERIALX_BUILD_SHARED_LIBS=ON
  @@ -46,18 +39,12 @@ jobs:
  python: 3.9
  upload_shaders: ON
  - - name: Linux_Clang_9_Python27
  - os: ubuntu-18.04
  - compiler: clang
  - compiler_version: "9"
  - python: 2.7
  - cmake_config: -DMATERIALX_PYTHON_VERSION=2 -DMATERIALX_BUILD_SHARED_LIBS=ON
  -
  - name: Linux_Clang_10_Python37
  os: ubuntu-20.04
  compiler: clang
  compiler_version: "10"
  python: 3.7
  + cmake_config: -DMATERIALX_BUILD_SHARED_LIBS=ON
  - name: Linux_Clang_13_Python39
  os: ubuntu-22.04
  