changelog: mark vinyl transaction isolation level support as breaking

The transaction isolation level used by Tarantool by default is
'best-effort' so adding support of transaction isolation levels to Vinyl
effectively switched the isolation level used by Vinyl transactions from
'read-committed' to 'best-effort', which potentially may cause more
conflicts. Let's mention it in the changelog and mark this change as
breaking.

Follow-up commit 588170a7 ("vinyl: implement transaction isolation
levels")
Follow-up #5522

NO_DOC=changelog
NO_TEST=changelog