ci(runner): update to ubuntu 20.04

ubuntu 18.04 runner is deprecated

.github/workflows/check.yml

@@ -168,7 +168,7 @@ jobs:
                 vimtag: v8.0.0184
               - vimbin: vim
                 vimtag: v8.0.1453
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     env:
         VIM_BIN: ${{ matrix.vimbin }}
         VIM_TAG: ${{ matrix.vimtag }}

bundle/vim-go/.github/workflows/git.yml

@@ -3,7 +3,7 @@ on: [pull_request]
 
 jobs:
   autosquash-commits-integrated:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v2.0.0

bundle/vim-go/.github/workflows/test.yml

@@ -3,7 +3,7 @@ on: [push, pull_request]
 jobs:
   lint:
     name: lint
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
     - name: set up python
       uses: actions/setup-python@v2.1.4
@@ -23,7 +23,7 @@ jobs:
       run: $GITHUB_WORKSPACE/scripts/lint vim-8.2
   test:
     name: test
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
       matrix: