Add schema_version to main index mapping

What does this MR do and why?

Related to #214601 (closed)

Adding schema_version to index mapping for the main index (contains, project, wiki_blobs, and blob document types).

Note: We would like to have a schema_version field available for future migrations in all document types.

This is prep work to be able to easily backfill traversal_ids for projects which will be needed for the IndexIntegrityWorker. The IndexIntegrityWorker (dev in progress) will need to pull all projects for a root namespace and I need the traversal_ids field populated for that. Currently it's only being populated for blobs and wiki_blobs.

Screenshots or screen recordings

N/A

How to set up and validate locally

prerequisites

setup Elasticsearch in gdk

check config.rb changes

1. create an index from scratch: bundle exec rake gitlab:elastic:index
2. verify the new mapping is there and has data (tests the config.rb changes) GET http://localhost:9200/gitlab-development/_search

{
	"query": {
		"bool": {
			"must": [
				{
					"term": {
						"type": {
							"value": "project"
						}
					}
				}
			]
		}
	}
}

migration changes

1. checkout the master branch
2. recreate the indexes from scratch : bundle exec rake gitlab:elastic:index
3. add a new project
4. verify that it doesn't throw an error and indexes properly (without the new field)
5. stop rails-background-jobs to prevent it from running the migration automatically: gdk stop rails-background-jobs
6. start rails console
7. run the migration in the rails console
8. verify that the schema_version field exists in the mappings: GET http://localhost:9200/gitlab-development/_mappings
9. add a new or update an existing project and run the indexing manually: Elastic::ProcessBookkeepingService.new.execute
10. verify that the schema_version field exists and is populated for the project in the search response GET http://localhost:9200/gitlab-development/_search

{
	"query": {
		"bool": {
			"must": [
				{
					"term": {
						"type": {
							"value": "project"
						}
					}
				}
			]
		}
	}
}

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