ci: correctly checkout vim-flavor using https://

Currently, the CI throws:

The unauthenticated git protocol on port 9418 is no longer supported.
Please see https://github.blog/2021-09-01-improving-git-protocol-security-github/ for more information.

So update vim-flavor to 4.0.1 which uses https:// links instead of
un-authenticated git protocol. Unfortunately, we have to update ruby to
version 3.0 with that.

Let's see if this works.

.github/workflows/ci.yml

@@ -27,9 +27,9 @@ jobs:
       uses: actions/checkout@master
 
     - name: Setup Ruby
-      uses: actions/setup-ruby@v1
+      uses: ruby/setup-ruby@v1
       with:
-        ruby-version: '2.6'
+        ruby-version: '3.0'
 
     - name: Setup Bundle
       run: |

Gemfile

@@ -1,2 +1,2 @@
 source 'https://rubygems.org'
-gem 'vim-flavor', '~> 2.2.2'
+gem 'vim-flavor', '~> 4.0.1'