Given that the switch back will be in September, let's upgrade s2 to fully Buster and MariaDB 10.4
[x] Sanitarium hosts on both DCs (already done https://phabricator.wikimedia.org/T275112 https://phabricator.wikimedia.org/T272008)
[x] Sanitarium masters on both DCs (https://phabricator.wikimedia.org/T280492) - ensure sanitarium replicates from a 10.4 host.
[x] Available backup source on the standby DC (all sections have sources everywhere now)
[] Reimage dbprov1002 from stretch to buster, reorganize backup generation
[] Switchover backup generation standby DC (to be confirmed by @jcrespo)
[] Candidate master on the standby DC
[] Master on the standby DC
[] Candidate master on the primary DC
[x] Available backup source on the primary DC (to be confirmed by @jcrespo)
[] Reimage dbprov2002 from stretch to buster, reorganize backup generation
[] Switchover backup generation Primary DC (to be confirmed by @jcrespo)
[] Switchover on the primary DC to promote a Buster+10.4 host to master:
[] Upgrade the old master and make it a candidate master.
[] Cleanup (remove) old backup sources from both DCs
Please read the [[ https://docs.google.com/document/d/1tYfOYROB_XjD0x_vrCRn8KpjCPUflCDuhKz8cXJvtFk/edit?ts=607fb79e | doc ]] about procedure for more details.