Warn against the terms blacklist / whitelist

These terms carry racial connotations and are inappropriate. While
we cannot introduce testing for these words at the error level,
because they are used in the UI and API at this time, we can start
testing at the warning level and increase to error level once the
terms are removed from the UI.

doc/.vale/gitlab/SubstitutionWarning.yml

@@ -10,7 +10,9 @@ level: warning
 ignorecase: true
 swap:
   admin: administrator
+  blacklist(ed|ing)?: denylist
   config: configuration
   distro: distribution
   info: information
   repo: repository
+  whitelist(ed|ing)?: allowlist