Add class to migration worker and record logs

What does this MR do and why?

Adds class to structured logging for Elastic::MigrationWorker and Elastic::MigrationRecord

Screenshots or screen recordings

N/A

How to set up and validate locally

1. tail the elasticsearch.log - tail -f <gdk_dir>/gitlab/log/elasticsearch.log
2. run the migration_worker_spec.rb - bundle exec rspec ee/spec/workers/elastic/migration_worker_spec.rb
3. verify in the log that the class field is set for all log entries

example

{"severity":"INFO","time":"2023-03-09T21:25:16.614Z","correlation_id":"c3016c58da3e3c78bc7363850610a303","class":"Elastic::MigrationWorker","message":"MigrationWorker: migration[ApplyMaxAnalyzedOffset] updating with completed: false","job_status":"running","queue":"default","jid":null}

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.