Add strategy with serializable transaction

[leoarnold]

Sometimes you need to use a serializable transaction, e.g. to implement a pessimistic "find or create"
as explained in this blog post.

In this case, we cannot clean with the Transaction strategy because we cannot nest a serializable transaction into a normal one.

We could clean with truncation, but that slows test down dramatically.

Starting with Rails v8.1.0, we can nest a transaction into transaction of the same isolation level.

This adds a cleaning strategy based on a serializable transaction.

[leoarnold]

Note: The following PRs should be merged for this one to pass CI:

• Test against Rails 8.1 #131
• Camelize strategy name in shared examples database_cleaner#725