Remove last traces of Travis CI

* The build status badge is now for GitHub Actions.
* The documentation now mentions GitHub instead.
* Warnings in the YAML file have been fixed or ignored.

test/script/custom-linting-rules

@@ -143,8 +143,8 @@ check_errors '\(!=.\?\|isnot\) type({})' "Use 'isnot v:t_dict' instead"
 check_errors '\(!=.\?\|isnot\) type(function([^)]\+))' "Use 'isnot v:t_func' instead"
 
 # Run a Python script to find lines that require padding around them.  For
-# users without Python installed, we'll skip these checks. Travis CI will run
-# the script.
+# users without Python installed, we'll skip these checks. GitHub Actions will
+# run the script.
 if command -v python > /dev/null; then
     if ! test/script/block-padding-checker "$directory"/**/*.vim; then
         RETURN_CODE=1