Add missing terra 0.21.0 release notes

In Qiskit/qiskit#8320 a mistake was fixed where a release note for
0.21.0 was committed to the wrong location in the repository. This
prevented the release notes from being shown when reno was called to
build the rst page. This commit adds the release notes to the published
copy now that this has been caught so the release notes are included
with the others in 0.21.0.

docs/release_notes.rst

@@ -685,6 +685,23 @@ Upgrade Notes
   See `#7969 <https://github.com/Qiskit/qiskit-terra/pull/7949>`__
   for details.
 
+.. releasenotes/notes/0.21/remove-hard-time-limit-vf2-be83830ecc71f72c.yaml @ b'5a8bb42ec02753dce68ea0d28986453d07d071b2'
+
+- 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.
+
 
 .. _Release Notes_0.21.0rc1_Deprecation Notes:
 
@@ -941,6 +958,15 @@ Bug Fixes
   not affect all the qubits they are defined over.
   Fixed `#8224 <https://github.com/Qiskit/qiskit-terra/issues/8224>`__.
 
+.. releasenotes/notes/0.21/remove-hard-time-limit-vf2-be83830ecc71f72c.yaml @ b'5a8bb42ec02753dce68ea0d28986453d07d071b2'
+
+- 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.
+
 
 Aer 0.10.4
 ==========