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

changelogs/unreleased/gh-5522-vy-tx-isolation-level.md

 ## feature/vinyl
 
-* Added support of transaction isolation levels for the Vinyl engine.
-  The `txn_isolation` option passed to `box.begin()` now has the same
-  effect for Vinyl and memtx (gh-5522).
+* **[Breaking change]** Added support of transaction isolation levels for the
+  Vinyl engine. The `txn_isolation` option passed to `box.begin()` now has the
+  same effect for Vinyl and memtx. Note, this effectively changes the default
+  isolation level of Vinyl transactions from 'read-committed' to 'best-effort',
+  which may cause more conflicts (gh-5522).