ci: Update distro to Ubuntu 21.04

Let's see if this fixes our aarch64 failures.
4 jobs for ci-fix in 3 minutes and 14 seconds (queued for 1 second)
Status Job ID Name Coverage
  Container
failed #10952210
aarch64
build-container-aarch64

00:00:09

passed #10952209
build-container-x86_64

00:03:13

 
  Build
skipped #10952212
aarch64
build-aarch64
skipped #10952211
build-x86_64
 
Name Stage Failure
failed
build-container-aarch64 Container
$ if [[ -z "$CI_REGISTRY_IMAGE" ]] # collapsed multi-line command
$ podman login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
/bin/sh: eval: line 136: podman: not found
Uploading artifacts for failed job
Uploading artifacts...
WARNING: container-build-report.xml: no matching files
ERROR: No files to upload
Cleaning up file based variables
ERROR: Job failed: exit code 127