Create Elasticsearch migration to update code index options

As discussed in !96785 (comment 1119501226) it should be possible to reindex all code in a long-running migration that requires zero index pausing via the following steps:

1. Introduce a new field like blob.content_2_0 with the new settings
2. Update searches to search both blob.content and blob.content_2_0
3. Change our writes to set blob.content_2_0 and blank out blob.content. It would probably be too costly to set both
4. Ship a migration which does a reindex by query in batches to copy the content over and blank out the old field
5. When the migration is finished we could deprecate the blob.content field somehow but probably it will need to be in a major release for backwards compatibility