Retire Ubuntu 18.04 CI builder

[behlendorf]

Motivation and Context

The GitHub-hosted Ubuntu 18.04 has been deprecated and will be entirely unsupported as of April 2023. Leading up to this there will be scheduled "brownouts" to encourage users to update their workflows.

Retiring these runners now also has the advantage of resolving the frequent CI failures on Ubuntu 18.04 due to hitting the maximum instance run time. This is believed to be caused by these running using older, slower, hardware.

Description

This commit retires our use of the GitHub-hosted Ubuntu 18.04 runners in advance of their removal.

How Has This Been Tested?

Locally inspected, will be tested by the CI.

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[szubersk]

The reason CI hits the timeout is (logs from this PR)

[ 9719.718092] ZTS run /usr/share/zfs/zfs-tests/tests/functional/mmp/mmp_active_import                                             
[10030.729687] INFO: task zpool:1268006 blocked for more than 120 seconds.                                                         
[10030.736226]       Tainted: P           OE     5.15.0-1023-azure #29~20.04.1-Ubuntu                                              
[10030.743056] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.                                           
[10030.750482] task:zpool           state:D stack:    0 pid:1268006 ppid:1259863 flags:0x00004000                                  
[10030.750488] Call Trace:                                                                                                         
[10030.750491]  <TASK>                                                                                                             
[10030.750495]  __schedule+0x2c2/0x8e0                                                                                             
[10030.750501]  schedule+0x69/0x110                                                                                                
[10030.750506]  cv_wait_common+0x157/0x260 [spl]                                                                                   
[10030.750518]  ? wait_woken+0x60/0x60                                                                                             
[10030.750524]  __cv_wait+0x15/0x20 [spl]                                                                                          
[10030.750531]  vdev_autotrim_stop_wait+0x67/0xe0 [zfs]                                                                            
[10030.750726]  vdev_autotrim_stop_all+0x35/0x50 [zfs]                                                                             
[10030.750889]  spa_export_common+0x327/0x5a0 [zfs]                                                                                

The bug above manifests itself more often on Ubuntu 18.04 (I downloaded 18.04 logs a couple of times from different PRs) than others but still.

[behlendorf]

Ah, I hadn't seen that. Was there anything else interesting in the log before that stack? We should get an issue open to at least track this.