Revert "Upgrade CI image to Ubuntu Jammy"

This reverts commit 05c997802b.

It seems we need to update our docker infrastructure on the host first
before upgrading to Ubuntu Jammy.

ci/images/README.adoc

@@ -11,7 +11,7 @@ $ docker build --no-cache -f <image-folder>/Dockerfile .
 For example
 
 ----
-$ docker build --no-cache -f ci-image/Dockerfile .
+$ docker build --no-cache -f spring-framework-ci-image/Dockerfile .
 ----
 
 To test run:

ci/images/ci-image/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:jammy-20221020
+FROM ubuntu:focal-20220922
 
 ADD setup.sh /setup.sh
 ADD get-jdk-url.sh /get-jdk-url.sh