Investigate change to Spring Data MongoDB's default UUID representation

[wilkinsona]

It's changed to STANDARD (from JAVA_LEGACY). We have a test that should have detected the change but it doesn't seem to be failing. We need to investigate what's happening here and:

1. Make sure that our default is aligned with Spring Data Mongo DB
2. Update the test if necessary so that it detects misalignment

If 2 is necessary, we may want to make the change on all supported branches.

[mp911de]

There's a follow-up spring-projects/spring-data-mongodb#5037

We've realized that our defaulting changes are too subtle in comparison to their impact on existing data. We consider backing off from defaults and consider requiring a configuration option instead to introduce a breaking change for users that make use of UUIDs and BigDecimal/BigInteger. While this creates some inconvenience, it seems a better approach than discovering on existing data that queries no longer work or worse, storing mixed data where other processes rely on a specific format. Realistically, integration tests happen quite often with empty databases and issues only show up when there is data already in a database.