ci: update docs_ci.yml, gen_vc.yml and module_docs.yml to use more modern runners

.github/workflows/docs_ci.yml

@@ -6,7 +6,7 @@ on: [push, pull_request]
 
 jobs:
   check-markdown:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     timeout-minutes: 5
     steps:
     - uses: actions/checkout@v2

.github/workflows/gen_vc.yml

@@ -9,7 +9,7 @@ on:
 
 jobs:
   build-vc:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
     - name: Build V

.github/workflows/module_docs.yml

@@ -8,7 +8,7 @@ on:
 
 jobs:
   build-module-docs:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v2
     - name: Build V