MONGOID-5576: Fix Github Actions unsupported Ubuntu version (#5560)

Author: johnnyshields

.github/workflows/test.yml

@@ -138,7 +138,6 @@ jobs:
           i18n:
           gemfile: gemfiles/rails-6.0.gemfile
           experimental: false
-
         - mongodb: '5.0'
           ruby: ruby-3.1
           topology: replica_set
@@ -162,7 +161,7 @@ jobs:
         - mongodb: '4.0'
           ruby: ruby-2.7
           topology: replica_set
-          os: ubuntu-18.04
+          os: ubuntu-20.04
           task: test
           driver: current
           rails:
@@ -172,15 +171,14 @@ jobs:
         - mongodb: '3.6'
           ruby: ruby-2.7
           topology: replica_set
-          os: ubuntu-18.04
+          os: ubuntu-20.04
           task: test
           driver: current
           rails:
           i18n:
           gemfile: Gemfile
           experimental: false
 
-
     steps:
     - name: repo checkout
       uses: actions/checkout@v2