chore: prepare pre-release 2.16.0rc0

Author: vchudnov-g

CHANGELOG.md

@@ -4,7 +4,7 @@
 
 [1]: https://pypi.org/project/google-api-core/#history
 
-## [2.16.0](https://github.com/googleapis/python-api-core/compare/v2.15.0...v2.16.0) (2023-12-12)
+## [2.16.0rc0](https://github.com/googleapis/python-api-core/compare/v2.15.0...v2.16.0rc0) (2023-12-12)
 
 
 ### Features

google/api_core/version.py

@@ -12,4 +12,4 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-__version__ = "2.16.0"
+__version__ = "2.16.0rc0"