Fix Elastic::MigrationWorker current_migration (2nd attempt)

What does this MR do and why?

Related to #340478 (closed)

This is a second attempt to fix a bug with Advanced Search migrations, it was originally introduced in: !69958 (merged) and reverted in: !70405 (merged). The revert occurred because new migrations were not being picked up. The original fix changed from looking at completed migrations to not completed migrations. New migrations do not have a record in the migrations index so they would never be found.

This MR fixes the bug in Elastic::MigrationWorker#current_migration by

• Removing the rescue from StandardError from Elastic::MigrationRecord.load_versions
• Move Elastic::MigrationWorker#current_migration back to using completed migrations
• Rescue from StandardError in Elastic::MigrationWorker#current_migration and return nil if an error occurs (note: this will catch Elasticsearch communication errors as well as errors where the migrations index is not found)
• Move the migration index exists check in the Elastic::MigrationWorker.perform method to the top, this is to avoid an infinite loop if the index doesn't exist since current_migration now returns nil in that scenario
• update specs for new behavior

Screenshots or screen recordings

N/A

How to set up and validate locally

You must have your gdk setup to use Elasticsearch

1. in rails console, run Elastic::MigrationRecord.load_versions(completed: true)
2. verify all of the migrations come back in an array
3. run gdk stop elasticsearch
4. in rails console, run Elastic::MigrationRecord.load_versions(completed: true)
5. verify an exception is raised

It's hard to replicate a communication error while using the Elastic::MigrationWorker. However, you can add a breakpoint to simulate Elasticsearch being down before the current_migration call is run by taking down Elasticsearch at the breakpoint.

1. run gdk start elasticsearch
2. add a breakpoint in the Elastic::MigrationWorker.perform method before the migration = current_migration line

3. in rails console, run Elastic::MigrationWorker.new.perform
4. when the breakpoint is hit, run gdk stop elasticsearch
5. verify that current_migration returns nil and doesn't raise an exception

MR acceptance checklist

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