Refactor Elastic::MigrationWorker current_migration

currently the current_migration method uses a negative condition to support new migrations, we should switch over to using .exclude? to improve code readability.

current

Elastic::DataMigrationService.migrations.find { |migration| !completed_migrations.include?(migration.version) }

proposed

Elastic::DataMigrationService.migrations.find { |migration| completed_migrations.exclude?(migration.version) }

The following discussion from !70429 (merged) should be addressed:

• @cwiesner started a discussion: (+2 comments)

  suggestion: (non-blocking) Instead of negating the condition, could we use exclude? instead?

        Elastic::DataMigrationService.migrations.find { |migration| completed_migrations.exclude?(migration.version) }