tests: Replace vim 7.4 with vim 9.0 in the matrix

Vim 8.0 was released in 2016, which should be sufficient backwards
compatibility. We still support 7.4, but will not run it in the CI going
forwards too keep the test matrix sane.

Author: tbabej

.github/workflows/tests.yaml

@@ -13,10 +13,10 @@ jobs:
           - TASK_VERSION: v2.5.3
           - TASK_VERSION: v2.6.0
           - TASK_VERSION: develop
-          - VIM_VERSION: v7.4.2367
           - VIM_VERSION: v8.0.1850
           - VIM_VERSION: v8.1.2424
           - VIM_VERSION: v8.2.5172
+          - VIM_VERSION: v9.0.0000
           - VIMWIKI_VERSION: dev
           - PYTHON_VERSION: 3.6
           - PYTHON_VERSION: 3.7