travis.yml: Update the s390x jobs to Ubuntu Focal

QEMU will soon drop the support for Ubuntu 18.04, so let's update
the Travis jobs that were still using this version to 20.04 instead.

While we're at it, also remove an obsolete comment about Ubuntu
Xenial being the default for our Travis jobs.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220221153423.1028465-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

.travis.yml

-# The current Travis default is a VM based 16.04 Xenial on GCE
-# Additional builds with specific requirements for a full VM need to
-# be added as additional matrix: entries later on
 os: linux
 dist: focal
 language: c
@@ -190,7 +187,7 @@ jobs:
 
     - name: "[s390x] GCC check-tcg"
       arch: s390x
-      dist: bionic
+      dist: focal
       addons:
         apt_packages:
           - libaio-dev
@@ -233,7 +230,7 @@ jobs:
 
     - name: "[s390x] GCC (other-softmmu)"
       arch: s390x
-      dist: bionic
+      dist: focal
       addons:
         apt_packages:
           - libaio-dev
@@ -263,10 +260,11 @@ jobs:
 
     - name: "[s390x] GCC (user)"
       arch: s390x
-      dist: bionic
+      dist: focal
       addons:
         apt_packages:
           - libgcrypt20-dev
+          - libglib2.0-dev
           - libgnutls28-dev
           - ninja-build
       env:
@@ -274,7 +272,7 @@ jobs:
 
     - name: "[s390x] Clang (disable-tcg)"
       arch: s390x
-      dist: bionic
+      dist: focal
       compiler: clang
       addons:
         apt_packages: