Bump gitlab-elasticsearch-indexer to v2.6.0

Use new version of the elasticsearch indexer to support
indexing blobs with path > 512 bytes

GITLAB_ELASTICSEARCH_INDEXER_VERSION

-2.5.0
+2.6.0

ee/changelogs/unreleased/5870-elasticsearch-id-is-too-long-must-be-no-longer-than-512-bytes.yml

+---
+title: Support Indexing files in Elasticsearch with paths longer than 512 bytes
+merge_request: 37834
+author:
+type: fixed

ee/spec/lib/gitlab/elastic/indexer_spec.rb

@@ -373,6 +373,21 @@ RSpec.describe Gitlab::Elastic::Indexer do
     end
   end
 
+  context 'when a file path is larger than elasticsearch max size of 512 bytes', :elastic do
+    let(:long_path) { "#{'a' * 1000}_file.txt" }
+
+    before do
+      project.repository.create_file(user, long_path, 'Large path file contents', message: 'long_path.txt', branch_name: 'master')
+
+      index_repository(project)
+    end
+
+    it 'indexes the file' do
+      files = indexed_file_paths_for('file')
+      expect(files).to include(long_path)
+    end
+  end
+
   def expect_popen
     expect(Gitlab::Popen).to receive(:popen)
   end