circleci: Trade Ubuntu xenial for focal

And bionic inherits the "oldest Ubuntu" 32bit setting.

.circleci/config.yml

@@ -162,7 +162,7 @@ jobs:
         description: the Linux distribution (debian|ubuntu)
         type: string
       release:
-        description: the release name (stretch|buster|xenial|bionic)
+        description: the release name (stretch|buster|bionic|focal)
         type: string
       extra_conf:
         description: platform-specific configure arguments
@@ -328,9 +328,9 @@ workflows:
           release: buster
           extra_conf: --enable-asan --enable-ubsan --enable-workspace-emulator
       - distcheck:
-          name: distcheck_ubuntu_xenial
+          name: distcheck_ubuntu_bionic
           dist: ubuntu
-          release: xenial
+          release: bionic
           extra_conf: CFLAGS='-g -O2 -m32'
       - distcheck:
           name: distcheck_alpine
@@ -361,7 +361,7 @@ workflows:
             parameters:
               platform:
                 - ubuntu:bionic
-                - ubuntu:xenial
+                - ubuntu:focal
                 - debian:bullseye
                 - debian:buster
                 - debian:stretch