Change 'warning' to 'suggestion'

When we changed the reporting level of this rule, the commented
section at the beginning of the file wasn't changed to match.

doc/.linting/vale/styles/gitlab/FutureTense.yml → doc/.vale/gitlab/FutureTense.yml

 ---
-# Warning: gitlab.FutureTense
+# Suggestion: gitlab.FutureTense
 #
 # Checks for use of future tense in sentences. Present tense is preferred as
 # much as possible.