I searched the GitHub PRs for UUID using this query to find when it was introduced:
There was also a PR to fix some issues:
Both were merged in 6.0.0.
In the first PR I found the migration:
app/migrations/Version20210211081531.php
Then there are two migrations in the second one:
app/migrations/Version20210818090322.php
app/migrations/Version20210819201726.php
If you’ve done an upgrade and the migrations were processed, it should have migrated your database and completed the necessary changes.
I alos searched the issue queue for UUID and found this issue: Missing `uuid` field in Lead entity after updating to 6.0.0-rc · Issue #14760 · mautic/mautic · GitHub which was fixed with the later PR:
Also released with 6.0.0.