Werk #15958: Custom extensions: Avoid duplicate graphs in the user interface due to `graph_info.append`

This werk only concerns developers of custom Checkmk extensions.

There are currently two ways for registering custom graph definitions:

graph_info["GRAPH_ID"] = GRAPH_DEFINITION

and

graph_info.append(GRAPH_DEFINITION)

Using the latter might have lead to duplicate graphs in the user interface. As of this werk, this is fixed.

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