Run CI on latest Ubuntu (prev version deprecated)

Author: djordjelacmanovic

.github/workflows/lint.yml

@@ -8,7 +8,7 @@ on:
 
 jobs:
   standardrb:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
 
     steps:
       - uses: actions/checkout@v2
@@ -19,7 +19,7 @@ jobs:
         run: bundle exec standardrb
 
   rubocop-rspec:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
 
     steps:
       - uses: actions/checkout@v2

.github/workflows/spec.yml

@@ -8,7 +8,7 @@ on:
 
 jobs:
   rake-spec:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     strategy:
       matrix:
         ruby: [2.5, 2.6, 2.7, 3.0, 3.1]