fix: issue when cluster is moved to a different shard

[lplazas]

Fixes: http://github.com/argoproj/argo-cd/issues/19854

When using any sharding algorithm that moves shards with some frequency this issue becomes more apparent.

Simple steps to reproduce:

1. When a cluster is moved for any reason (new cluster added, new app added, etc) you will see in the logs something like Cluster https://XYZ.gr7.us-east-1.eks.amazonaws.com has changed shard from 4 to 6
2.1 handleModEvent() will be triggered on shard 4:
It will go into if !c.canHandleCluster(newCluster) { and delete the cluster from the local cache: https://github.com/argoproj/argo-cd/blob/master/controller/cache/cache.go#L833
2.2 handleModEvent() will be triggered on shard 6:
As shard 6 hasn't had newCluster.Server in its cache before, ok is set to false and the entire logic of handleModEvent is skipped and the cluster is never added to this shard's cache.

	c.clusterSharding.Update(oldCluster, newCluster)
	c.lock.Lock()
	cluster, ok := c.clusters[newCluster.Server]
	c.lock.Unlock()
	if ok { // THIS IS COMPLETELY SKIPPED, THE FUNCTION DOES NOTHING 

This fix checks if the cluster is not in local cache, wether it should be added. The call to getSyncedCluster populates the local cache as it calls getCluster that sets the entry in the cache: https://github.com/argoproj/argo-cd/blob/master/controller/cache/cache.go#L479

I don't have the time to look into adding tests for a few days so if someone can guide me to the right place to add them it will save me a lot of time.

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