Migrate from ethers v5 to ethers v6

[Siegrift]

Closes #177

Rationale

I followed https://docs.ethers.org/v6/migrating/ and tried to advance by small steps (that's why so many commits). Usually going from file to file, understanding the TS issue and fixing it globally. There were a few gotchas worth mentioning:

1. Multiple things are not mentioned in the migration guide
2. Be extra careful when deriving sponsor wallet. Especially if these are ported to commons (maybe as part of Migrate to ethers@6 commons#29)
3. Working with Hardhat is considerably slower. Seems like an internal change in the underlying provider implementation.
4. 379e0d0 fixes a breaking change with how ethers "ArrayObjects" work. In ethers v5 when you fetched an event or called a function, you received back for example:

Promise<[BigNumber, number] & {
    value: BigNumber;
    timestamp: number;
}>

when you did Object.keys(suchResponse) you got back ['0', '1', 'value', 'timestamp']. In ethers v6, what you get is ['0', '1'] (the named keys are not own properties). This breaks utilities such as omit.

Quality assurance

Apart from fixing all of the tests, I ran Airseeker locally for a few hours and it updated the feeds correctly so I think all is fine. I

TODO

• Write PR description
• Fix TODOs in code
• Consider using BigInt literal (e.g. 123n) instead of BigInt constructor
• Fix CI test
• Run e2e local test flow
• Ensure that ethers.Wallet.fromPhrase derives the same wallet as ethers.Wallet.fromMnemonic in ethers v5
• Fix tests

[aquarat]

LGTM 👍 only a very very minor comment around BigInt literals.
This was good for me as I have a better idea now of the differences between ethers v5 and v6 🚀