CI: drop Ubuntu 20.04, add AlmaLinux 8

[AkihiroSuda]

The ubuntu-20.04 runner is being deprecated on GHA:

• The Ubuntu 20.04 Actions runner image will begin deprecation on 2025-02-01 and will be fully unsupported by 2025-04-01 actions/runner-images#11101

AlmaLinux 8 can be still used for running the tests with cgroup v1.

[AkihiroSuda]

=== FAIL: cmd/nerdctl/container TestRunDevice (0.44s)
    container_run_cgroup_linux_test.go:236: assertion failed: error is not nil: loopback setup failed ([losetup --find --show /tmp/containerd-test-loopback3931357228]): stdout="", stderr="losetup: /tmp/containerd-test-loopback3931357228: failed to set up loop device: No such file or directory\n": exit status 1

https://github.com/containerd/nerdctl/actions/runs/13431403718/job/37523922230?pr=3904

[apostasie]

=== FAIL: cmd/nerdctl/container TestRunDevice (0.44s)
    container_run_cgroup_linux_test.go:236: assertion failed: error is not nil: loopback setup failed ([losetup --find --show /tmp/containerd-test-loopback3931357228]): stdout="", stderr="losetup: /tmp/containerd-test-loopback3931357228: failed to set up loop device: No such file or directory\n": exit status 1

https://github.com/containerd/nerdctl/actions/runs/13431403718/job/37523922230?pr=3904

@AkihiroSuda

Probably need to yank out snapd?

      - name: "Remove snap loopback devices (conflicts with our loopback devices in TestRunDevice)"
        run: |
          sudo systemctl disable --now snapd.service snapd.socket
          sudo apt-get purge -y snapd
          sudo losetup -Dv
          sudo losetup -lv

[apostasie]

Other two CI failures are the recent regression on TestStop (#3908) and some likely unrelated Compose/Stargz race.