ci: Downgrade docs docker image Ubuntu version to focal

The currently used version, impish, is not updatable anymore since July
the 14th and we don't have a newer version in our repository. Downgrade
this to focal to fix the container image builds.

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>

.oniro-ci/containers/docs-builder/Dockerfile

 # SPDX-FileCopyrightText: Huawei Inc.
 # SPDX-License-Identifier: Apache-2.0
 
-FROM ubuntu:21.10
+FROM ubuntu:20.04
 
-COPY --chown=root:root ppa/zyga-ubuntu-oh-tools-impish.list /etc/apt/sources.list.d/
+COPY --chown=root:root ppa/zyga-ubuntu-oh-tools-focal.list /etc/apt/sources.list.d/
 COPY --chown=root:root ppa/zyga-ubuntu-oh-tools.gpg /etc/apt/trusted.gpg.d/
 
 ARG DEBIAN_FRONTEND="noninteractive"

.oniro-ci/containers/docs-builder/ppa/zyga-ubuntu-oh-tools-impish.list → .oniro-ci/containers/docs-builder/ppa/zyga-ubuntu-oh-tools-focal.list

 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: 2020 Huawei Inc.
-deb http://ppa.launchpad.net/zyga/oh-tools/ubuntu/ impish main
-# deb-src http://ppa.launchpad.net/zyga/oh-tools/ubuntu/ impish main
+deb http://ppa.launchpad.net/zyga/oh-tools/ubuntu/ focal main
+# deb-src http://ppa.launchpad.net/zyga/oh-tools/ubuntu/ focal main