Downgraded Ubuntu to 18.04

Getting errors with LibGit2Sharp and this suggested it doesn't support beyond 18.04: libgit2/libgit2sharp#1703 (comment)

Author: markwhitaker

.github/workflows/build-and-test.yml

@@ -8,7 +8,7 @@ on:
 jobs:
   build-and-test:
 
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-18.04
 
     steps:
       - name: Checkout

.github/workflows/publish-to-nuget.yml

@@ -7,7 +7,7 @@ on:
 jobs:
   build-and-test:
 
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-18.04
 
     steps:
       - name: Checkout
@@ -28,7 +28,7 @@ jobs:
 
     needs: build-and-test
 
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-18.04
 
     steps:
       - name: Checkout