Advanced Search: Use namespace_ancestry for issues

What does this MR do?

This change is behind the elasticsearch_use_group_level_optimization feature flag

This is the second step of #335825 (closed)

1. Advanced Search: Add namespace_ancestry_ids to issues index
2. (this MR) Using the namespace_ancestry_ids field to improve group issues searches

This MR improves the group issues search by utilizing prefix search against the namespace_ancestry_ids field added in !70042 (merged). Instead of passing a list of projects in order to complete a group-level search, we'll use a wildcard (prefix) search. Here's the original technical proposal: #335825 (comment 625116161). The only change since then is that now we have a trailing separator (1-3-4 becomes 1-3-4-)

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

Example below:

1. Get your gdk setup to work with Elasticsearch and create your index from scratch
2. Please ensure that the :redo_backfill_namespace_ancestry_ids_for_issues migration is completed before continuing

[1] pry(main)> Elastic::DataMigrationService.migration_has_finished?(:redo_backfill_namespace_ancestry_ids_for_issues)
=> true

1. Enable the Performance Bar

2. Visit /search and search for issues inside a group (not in a project), ensure that Advanced Search is working and you have results for the query

3. Enable the feature flag

   Feature.enable(:elasticsearch_use_group_level_optimization)

4. Wait a minute for the feature flag cache to update and refresh the search page

5. Ensure that there are still the same results

6. Confirm that in the performance bar behind ES calls you can see "prefix": { "namespace_ancestry_ids" as a part of the query

   Click to see an example query

    {
      "query":{
        "bool":{
          "must":[
            {
              "simple_query_string":{
                "_name":"issue:match:search_terms",
                "fields":[
                  "iid^3",
                  "title^2",
                  "description"
                ],
                "query":"update",
                "lenient":true,
                "default_operator":"and"
              }
            }
          ],
          "filter":[
            {
              "term":{
                "type":{
                  "_name":"doc:is_a:issue",
                  "value":"issue"
                }
              }
            },
            {
              "bool":{
                "should":[
                  {
                    "prefix":{
                      "namespace_ancestry_ids":{
                        "_name":"issue:authorized:namespace:ancestry_filter:descendants",
                        "value":"22-"
                      }
                    }
                  }
                ]
              }
            }
          ]
        }
      },
      "highlight":{
        "fields":{
          "iid":{
    
          },
          "title":{
    
          },
          "description":{
    
          }
        },
        "number_of_fragments":0,
        "pre_tags":[
          "gitlabelasticsearch→"
        ],
        "post_tags":[
          "←gitlabelasticsearch"
        ]
      }
    }
   

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related to #335825 (closed)