fix typo in env, undo ubuntu os changes, set gcc version in CI to 6

this system object of this version is being provided by gcc, and the runners no longer come prepackaged with it since its old (?)

.github/workflows/CI.yml

@@ -40,7 +40,7 @@ jobs:
   build-and-test-unix:
     strategy:
       matrix:
-        os: [ubuntu-20.04, ubuntu-latest, macos-latest]
+        os: [ubuntu-latest, macos-latest]
     runs-on: ${{ matrix.os }}
     # skip scheduled runs on main from forks
     if: ${{ !( github.repository != 'ReactionMechanismGenerator/RMG-Py' && github.event_name == 'schedule' ) }}
@@ -52,6 +52,12 @@ jobs:
     steps:
       - uses: actions/checkout@v3
 
+      - name: Set up gcc-6
+        uses: egor-tensin/setup-gcc@v1
+        with:
+          version: 6
+          platform: x64
+
       # configures the mamba environment manager and builds the environment
       - name: Setup Mambaforge Python 3.7
         uses: conda-incubator/setup-miniconda@v2

environment.yml

@@ -13,7 +13,7 @@
 #
 # Changelog:
 # - May 15, 2023 Added this changelog, added inline documentation,
-#   made depdency list more explicit (@JacksonBurns).
+#   made dependency list more explicit (@JacksonBurns).
 #
 name: rmg_env
 channels:
@@ -22,9 +22,6 @@ channels:
   - conda-forge
   - cantera
 dependencies:
-# Fixing old versions of things that seem to break otherwise
-  - libstdcxx-ng < 13
-
 # System-level dependencies - we could install these at the OS level
 # but by installing them in the conda environment we get better control
   - cairo