Feat: OptimismMintableUpgradableERC20

[sander2]

The second commit contains the changes compared to the default contract.

Changes:

• removed args from constructor
• switch from ERC20 to ERC20Upgradeable
• Make immutable variables mutable (but keep the naming convention as if they were still immutable)

[changeset-bot]

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Latest commit: b8eeb31

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