fix: dev/tasks/linux-packages/apache-arrow-apt-source/apt/ubuntu-foca…

…l/Dockerfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-UBUNTU2004-SHADOW-5879186
- https://snyk.io/vuln/SNYK-UBUNTU2004-SHADOW-5879186
- https://snyk.io/vuln/SNYK-UBUNTU2004-SYSTEMD-3339226
- https://snyk.io/vuln/SNYK-UBUNTU2004-SYSTEMD-3339226
- https://snyk.io/vuln/SNYK-UBUNTU2004-XZUTILS-5854646

dev/tasks/linux-packages/apache-arrow-apt-source/apt/ubuntu-focal/Dockerfile

@@ -15,7 +15,7 @@
 # specific language governing permissions and limitations
 # under the License.
 
-FROM ubuntu:focal
+FROM ubuntu:20.04
 
 RUN \
   echo "debconf debconf/frontend select Noninteractive" | \