dataflow (fix): pin in to ubuntu:impish (GoogleCloudPlatform#7823)

It looks like on the newly released `ubuntu:jammy` which is what `ubuntu:latest` points to now, the available Python installation is Python 3.10. Even though Apache Beam is available in Python 3.10 through `pip`, there is no "official" support for it so there is no `apache/beam_python3.10_sdk` image.

So we're pinning to `ubuntu:impish` which is the latest ubuntu image with Python 3.9 support.

Co-authored-by: Maciej Strzelczyk 

Author: davidcavazos

dataflow/custom-containers/ubuntu/Dockerfile

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-FROM ubuntu:latest
+FROM ubuntu:impish
 
 WORKDIR /pipeline