The topology coordinator only marks a replaced node as LEFT
during the replace operation and actually removes it from the
group 0 config in cleanup_group0_config_if_needed. If this
function is called before raft has committed a replacing node as
a voter, it does not remove the replaced node from the group 0
config. Then, the coordinator can decide that it has no work to
do and starts sleeping, leaving us with an outdated config.
This behavior reduces group 0 availability and causes problems in
tests (see #14885). Also, it makes the coordinator's logic
confusing - it claims that it has no work to do when it has some
work to do. Therefore, we modify the coordinator so that it
removes the replaced node earlier in handle_topology_transition.
Fixes#14885Fixes#14975Closes#15009
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