ci: Drop older Vim versions from the test matrix

These vesions are being dropped from the matrix due to their age (5+
years) and due to the fact that the CI as the current build process
fails for them.

However, I would still expect them to work until we start relying on
modern (9.0+ or newer) Vim features.

Author: tbabej

.github/workflows/tests.yaml

@@ -14,8 +14,6 @@ jobs:
           - TASK_VERSION: v2.6.0
           - TASK_VERSION: v2.6.2
           # - TASK_VERSION: develop  # TODO: Requires adjustment to the build process
-          - VIM_VERSION: v8.0.1850
-          - VIM_VERSION: v8.1.2424
           - VIM_VERSION: v8.2.5172
           - VIM_VERSION: v9.0.0000
           - VIM_VERSION: v9.1.1435