Upgrade RewardsSourceProxy's implementation to the new contract

[shahthepro]

Follow up of #297

The new contract is deployed here and the code is also verified on Etherscan.

Related PR: https://github.com/OriginProtocol/ousd-governance/pull/340

Pending things:

• [x] Call claimGovernance() on the new contract from the multi-sig wallet
• [x] Upgrade the proxy's implementation to the new contract

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Command used to deploy the contract:

brownie run deploy_002_upgrade_rewards_source --network mainnet

stdout of the command

[franckc]

A transaction has been submitted to the multi-sig. Waiting on review from the signers.

[franckc]

This is done.