Update README in v2.0 branch

- Removes notice of branch changes in README
- Modifies wording in the 'Contributing' section of the README
  to reflect Airship 2.0 release date
- Amends .gitreview to point at the v2.0 branch as default

Change-Id: I403d4bc453e6974eaa2c7cf02ab45280dbbd641a

.gitreview

@@ -2,4 +2,4 @@
 host=review.opendev.org
 port=29418
 project=airship/treasuremap.git
-defaultbranch=v2
+defaultbranch=v2.0

README.md

@@ -6,8 +6,9 @@ pre-configured workloads on baremetal and virtual infrastructure.
 
 ## Contributing
 
-Airship 2.0 is under active development and welcomes new developers! Please
-read our
+Airship 2.0 launched in April 2021, with plans to introduce enhanced functionality
+in subsequent releases. New developers are welcome to join and contribute to the
+evolution of Airship. Please read our
 [developer guide](https://docs.airshipit.org/airshipctl/developers.html) to
 begin contributing.
 
@@ -17,11 +18,3 @@ We also encourage new contributors and operators alike to join us in our
 
 You can learn more about Airship on the [Airship
 wiki](https://wiki.openstack.org/wiki/Airship).
-
-## Please Note:
-The branching structure of this repository will be changing to the following:
-* The 'v2' branch will become the 'master' branch
-* The current 'master' branch will become a new 'v1.9' branch
-* The current 'v1.8' branch will remain the same
-
-These changes are planned to take effect on Thursday, April 1, 2021, or slightly thereafter.