[doc] Update Ubuntu to 22.04

This updates documentation to reflect the switch to
Ubuntu 22.04 (jammy) in the CI:
https://review.opendev.org/c/openstack/devstack/+/860795

Change-Id: I8bee430029dcc719629bd92451c2791571f8a30c

Author: kopecmartin

doc/source/index.rst

@@ -40,7 +40,7 @@ Start with a clean and minimal install of a Linux system. DevStack
 attempts to support the two latest LTS releases of Ubuntu, the
 latest/current Fedora version, CentOS/RHEL/Rocky Linux 9 and OpenSUSE.
 
-If you do not have a preference, Ubuntu 20.04 (Focal Fossa) is the
+If you do not have a preference, Ubuntu 22.04 (Jammy) is the
 most tested, and will probably go the smoothest.
 
 Add Stack User (optional)

tools/install_pip.sh

@@ -139,7 +139,7 @@ if is_fedora && [[ ${DISTRO} == f* || ${DISTRO} == rhel9 ]]; then
     # recent enough anyway.  This is included via rpms/general
     : # Simply fall through
 elif is_ubuntu; then
-    # pip on Ubuntu 20.04 is new enough, too
+    # pip on Ubuntu 20.04 and higher is new enough, too
     # drop setuptools from u-c
     sed -i -e '/setuptools/d' $REQUIREMENTS_DIR/upper-constraints.txt
 else