leader-settings-changed firing before unit departure

[Mia Altieri]

Hi team. I had some weird behavior for this set up / situation

For Example. If you have these units:
```
A *
B
C
```
and `A` departs then you get these events hooks for each unit:
`A`: `leader-settings-changed` , `mongodb-storage-detaching`, `mongodb-relation-departed`(for A-B), `mongodb-relation-departed`(for A-C), `stop`, `remove`
`B`: `mongodb-relation-departed` (for A-B)
`C`: `mongodb-relation-departed` (for A-C),

According to @wallyworld and @rwcarlsen this is unexpected behaviour and should be looked into. As requested, [this](https://github.com/canonical/mongodb-operator/tree/DPE-65-relation-departed) is my repo/branch where I observed this behaviour. It can be observed by first creating the necessary environment according to [CONTRIBUTING.md](https://github.com/canonical/mongodb-operator/blob/main/CONTRIBUTING.md). Once the environment has been created you can reproduce this with:
```
# pack and deploy
charmcraft pack
juju deploy -n3 ./*charm

# in a separate session
juju debug-hooks <leader_unit>

# remove unit
juju remove-unit <leader_unit
```

[Ian Booth (wallyworld)]

Looking at the intent of the code, the unit agent has this logic:

// We want to run the leader settings hook if we're
// not the leader and the settings have changed.
if !localState.Leader && localState.LeaderSettingsVersion != remoteState.LeaderSettingsVersion {
    return opFactory.NewRunHook(hook.Info{Kind: hooks.LeaderSettingsChanged})
}

So we should only run leader-settings-changed if the unit is not the leader. Unit A was the leader at the time it was removed - and we are supposed to only take away leadership when unit A goes to Dead, ie after all hooks have been run. So it does seem something is amiss here.

[Ian Booth (wallyworld)]

The logic which determined what to run for leadership changes did not take account of a unit which was dying.

https://github.com/juju/juju/pull/13834

[Ian Booth (wallyworld)]

Looking at the code, this behaviour has been in place for a long time, probably ever since leadership was first implemented. So we'll need to consider if this is a change we can reasonably make in 2.9 since there may be charms which rely on this hook being fired during unit teardown.