Add neovim 0.6 to run-tests (#3998)

* Update test scripts
* Remove neovim 0.3 and 0.4
* Add neovim 0.6.1

Co-authored-by: Horacio Sanson <horacio@allm.inc>
Co-authored-by: w0rp <devw0rp@gmail.com>

.github/workflows/main.yml

@@ -29,8 +29,7 @@ jobs:
           - '--vim-80-only'
           - '--vim-82-only'
           - '--neovim-02-only'
-          - '--neovim-04-only'
-          - '--neovim-05-only'
+          - '--neovim-06-only'
           - '--linters-only'
     steps:
       - uses: actions/checkout@v2