Improve algortihms that compute tokens retirements and bridges per credit

[biwano]

• Use polygon-digital-carbon insead of staging-polygon-digital-carbon subgraph
• Migrate carbon metrics over to polygon-digital-carbon subgraph
• remove references to polygon-bridged-carbon subgraph
• Improve algortihms that compute tokens retirements and bridges per credit. In the current algorithm that computes tokens retirements and bridges per credit, we assume that the credit was bridged with only one token. The algorithm should take into account that multiple tokens can represent the same credit. It seems the information is missing in the Retire instances of the polygon-digital-carbon subgraph