Set version number to 1.2.0rc1 for first release candidate (#12867)

For the 1.2.0 release we're going to push release candidates prior to
the final to enable testing before we cut the final release. In
preparation for tagging the first release candidate this commit updates
the version string to indicate it's a release candidate. This commit
should be the final commit on main for 1.2.0rc1 and what gets tagged as
1.2.0rc1 post-merge.

docs/conf.py

@@ -32,7 +32,7 @@
 # The short X.Y version
 version = "1.2"
 # The full version, including alpha/beta/rc tags
-release = "1.2.0"
+release = "1.2.0rc1"
 
 language = "en"
 

qiskit/VERSION.txt

@@ -1 +1 @@
-1.2.0
+1.2.0rc1

releasenotes/notes/1.2/sabre-dense-trial-e3df737180281e35.yaml

@@ -0,0 +1,12 @@
+---
+features_transpiler:
+  - |
+    The :class:`.SabreLayout` transpiler pass has been updated to always run
+    an additional trial using the same algorithm as :class:`.DenseLayout` to
+    choose a starting point for Sabre's layout algorithm. The starting point
+    used by the layout algorithm can have a large influence on the quality of
+    the results. By default :class:`.SabreLayout` still start with
+    ``layout_trials`` random trials but in addition there will be a single
+    trial that uses the densest subgraph of the connectivity graph as the
+    starting point. This may yield better results in some cases especially
+    on cases with smaller circuits for larger coupling maps.