Elevate unclear antecedent rule to warning

doc/.vale/gitlab/UnclearAntecedent.yml

 ---
-# Suggestion: gitlab.UnclearAntecedent
+# Warning: gitlab.UnclearAntecedent
 #
 # Checks for words that need a noun for clarity.
 #
-# For a list of all options, see https://errata-ai.gitbook.io/vale/getting-started/styles
+# For a list of all options, see https://docs.errata.ai/vale/styles
 extends: existence
 message: "'%s' is not precise. Try rewriting with a specific subject and verb."
 link: https://docs.gitlab.com/ee/development/documentation/styleguide/word_list.html#this-these-that-those
-level: suggestion
+level: warning
 ignorecase: false
 tokens:
   - 'That is'