Remove Ubuntu 20.04 from CI (#167)

Author: HarrisonGrodin

.github/workflows/main.yml

@@ -17,7 +17,6 @@ jobs:
       fail-fast: false
       matrix:
         os:
-          - ubuntu-20.04
           - ubuntu-latest
           - macos-13
           - macos-14
@@ -36,11 +35,11 @@ jobs:
           - 110.99.7
         include:
           # SML/NJ 110.87 and 110.97
-          - os: ubuntu-20.04
+          - os: ubuntu-latest
             smlnj-version: 110.87
           - os: windows-latest
             smlnj-version: 110.87
-          - os: ubuntu-20.04
+          - os: ubuntu-latest
             smlnj-version: 110.97
           - os: windows-latest
             smlnj-version: 110.97
@@ -100,7 +99,6 @@ jobs:
       fail-fast: false
       matrix:
         os:
-          - ubuntu-20.04
           - ubuntu-latest
           - macos-13
         smlnj-version: