chore: set release version to 2.0.0 (#840)

* chore: release 2.0.0

* Update CHANGELOG.md

* chore: set version number to 2.0.0

Follow up to #829

Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>

Author: busunkim96

google/auth/version.py

@@ -12,4 +12,4 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-__version__ = "2.0.0b1"
+__version__ = "2.0.0"