Move stray 0.21.0 release note to a 0.21.0 release note file

releasenotes/notes/0.21/release-0.21.0-4a6c079c6301bde6.yaml

@@ -22,3 +22,27 @@ prelude: |
   Additionally, the transpiler has been improved to enable better quality
   outputs. This includes the introduction of new passes such as
   :class:`~.VF2PostLayout` and :class:`~.ToqmSwap`.
+upgrade:
+  - |
+    The preset pass managers generated by :func:`~.level_1_pass_manager`,
+    :func:`~.level_2_pass_manager`, and :func:`~.level_3_pass_manager` and used
+    by the :func:`~.transpile` function's ``optimization_level`` argument at
+    1, 2, and 3 respectively no longer set a hard time limit on the
+    :class:`~.VF2Layout` transpiler pass. This means that the pass will no
+    longer stop trying to find a better alternative perfect layout up until a
+    fixed time limit (100ms for level 1, 10 sec for level 2, and 60 sec for
+    level 3) as doing this limited the reproducibility of compilation when a
+    perfect layout was available. This means that the output when using the pass
+    might be different than before, although in all cases it would only change
+    if a lower noise set of qubits can be found over the previous output. If
+    you wish to retain the previous behavior you can create a custom
+    :class:`~.PassManager` that sets the ``time_limit`` argument on the
+    constructor for the :class:`~VF2Layout` pass.
+fixes:
+  - |
+    Fixed an issue with reproducibility of the :func:`~.transpile` function
+    when running with ``optimization_level`` 1, 2, and 3. Previously, under
+    some conditions when there were multiple perfect layouts (a layout that
+    doesn't require any SWAP gates) available the selected layout and output
+    circuit could vary regardless of whether the ``seed_transpiler`` argument
+    was set.