[MM-54495] Fix update post plugin hooks (#24602)

* Fix updatePost props and plugin hooks

* Add more tests

* Minor naming improvement

* Revert props related changes

* Fix test

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Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/app/post.go

@@ -702,7 +702,7 @@ func (a *App) UpdatePost(c *request.Context, receivedUpdatedPost *model.Post, sa
 	pluginContext := pluginContext(c)
 	a.ch.RunMultiHook(func(hooks plugin.Hooks) bool {
 		newPost, rejectionReason = hooks.MessageWillBeUpdated(pluginContext, newPost.ForPlugin(), oldPost.ForPlugin())
-		return receivedUpdatedPost != nil
+		return newPost != nil
 	}, plugin.MessageWillBeUpdatedID)
 	if newPost == nil {
 		return nil, model.NewAppError("UpdatePost", "Post rejected by plugin. "+rejectionReason, nil, "", http.StatusBadRequest)