Update doc tag for migration APIs

[lcawl]

Relates to #3518, elastic/elasticsearch#118291

This PR addresses the following errors returned from make lint-docs-errs:

18464:11  error  operation-tag-defined  Operation tags must be defined in global tags.                            paths./_migration/reindex/{index}/_cancel.post.tags[0]
18499:11  error  operation-tag-defined  Operation tags must be defined in global tags.                            paths./_create_from/{source}/{dest}.put.tags[0]
18524:11  error  operation-tag-defined  Operation tags must be defined in global tags.                            paths./_create_from/{source}/{dest}.post.tags[0]
18551:11  error  operation-tag-defined  Operation tags must be defined in global tags.                            paths./_migration/reindex/{index}/_status.get.tags[0]
18634:11  error  operation-tag-defined  Operation tags must be defined in global tags.                            paths./_migration/reindex.post.tags[0]

These errors were occurring because the APIs had tags that didn't exist in the global list of tags. I am assuming these APIs ought to appear in the "Migration" section (https://www.elastic.co/docs/api/doc/elasticsearch/group/endpoint-migration) and so am updating the tag accordingly. If they ought to be added to a different existing or new tag group, that's acceptable too.

[github-actions]

Following you can find the validation results for the APIs you have changed.

API	Status	Request	Response
migrate.cancel_reindex	🟢	2/2	2/2
migrate.create_from	🟢	2/2	2/2
migrate.get_reindex_status	🟢	1/1	1/1
migrate.reindex	🟢	2/2	2/2

You can validate these APIs yourself by using the make validate target.