Revert the Ubuntu version update (#42144)

Due to ECE compatibility issues we have to switch back to 20.04.

(cherry picked from commit e9a6565)

CHANGELOG.next.asciidoc

@@ -10,6 +10,9 @@ https://github.com/elastic/beats/compare/v8.8.1\...main[Check the HEAD diff]
 
 *Affecting all Beats*
 
+- Fix FQDN being lowercased when used as `host.hostname` {issue}39993[39993]
+- Beats won't log start up information when running under the Elastic Agent {40390}40390[40390]
+- Default Docker base image was reverted to Ubuntu 20.04 due to incompatability issues with glibc {pull}42144[42144]
 
 *Auditbeat*
 

dev-tools/packaging/packages.yml

@@ -159,7 +159,7 @@ shared:
   - &docker_spec
     <<: *binary_spec
     extra_vars:
-      from: '--platform=linux/amd64 ubuntu:24.04'
+      from: '--platform=linux/amd64 ubuntu:20.04'
       buildFrom: '--platform=linux/amd64 cgr.dev/chainguard/wolfi-base'
       user: '{{ .BeatName }}'
       linux_capabilities: ''
@@ -172,7 +172,7 @@ shared:
   - &docker_arm_spec
     <<: *docker_spec
     extra_vars:
-      from: '--platform=linux/arm64 ubuntu:24.04'
+      from: '--platform=linux/arm64 ubuntu:20.04'
       buildFrom: '--platform=linux/arm64 cgr.dev/chainguard/wolfi-base'
 
   - &docker_ubi_spec

dev-tools/packaging/templates/docker/Dockerfile.tmpl

@@ -57,7 +57,6 @@ RUN for iter in {1..10}; do \
 {{- end }}
 
 {{- if contains .from "ubuntu" }}
-RUN touch /var/mail/ubuntu && chown ubuntu /var/mail/ubuntu && userdel -r ubuntu
 
 RUN for iter in {1..10}; do \
         apt-get update -y && \