Fix docs build

Removes a non-existant file from the non-`// CONSOLE` list. If you
remove all the non-`// CONSOLE` snippets from a file it must be
removed from the whitelist for the build to pass. Removing the file
entirely counts as removing the snippets.

Related to #18160

docs/build.gradle

@@ -138,7 +138,6 @@ buildRestTests.expectedUnconvertedCandidates = [
   'reference/indices/shadow-replicas.asciidoc',
   'reference/indices/shard-stores.asciidoc',
   'reference/indices/update-settings.asciidoc',
-  'reference/indices/upgrade.asciidoc',
   'reference/ingest/ingest-node.asciidoc',
   'reference/mapping/dynamic/templates.asciidoc',
   'reference/mapping/fields/all-field.asciidoc',