Exclude expression indexes from reindexing

This excludes expression (aka "functional") indexes from reindexing due
to https://gitlab.com/gitlab-org/gitlab/-/issues/272997.

lib/gitlab/database/reindexing.rb

@@ -10,6 +10,7 @@ module Gitlab
       def self.candidate_indexes
         Gitlab::Database::PostgresIndex
           .regular
+          .where('NOT expression')
           .not_match("^#{ConcurrentReindex::TEMPORARY_INDEX_PREFIX}")
           .not_match("^#{ConcurrentReindex::REPLACED_INDEX_PREFIX}")
       end

spec/lib/gitlab/database/reindexing_spec.rb

@@ -24,7 +24,7 @@ RSpec.describe Gitlab::Database::Reindexing do
 
     it 'retrieves regular indexes that are no left-overs from previous runs' do
       result = double
-      expect(Gitlab::Database::PostgresIndex).to receive_message_chain('regular.not_match.not_match').with(no_args).with('^tmp_reindex_').with('^old_reindex_').and_return(result)
+      expect(Gitlab::Database::PostgresIndex).to receive_message_chain('regular.where.not_match.not_match').with(no_args).with('NOT expression').with('^tmp_reindex_').with('^old_reindex_').and_return(result)
 
       expect(subject).to eq(result)
     end