Feat/fix oz code inspector issues

[wshino]

I checked the contract vulnerabilities using OpenZeppelin Defender Code Inspector.
There are some issues, 1 high issue and 8 low issues.

High

1. Boolean literals used in complex expressions or as conditions

isDKIMPublicKeyHashValid in ECDSAOwnedDKIMRegistry
emailAuthMsg.proof.isCodeExist in EmailAuthMsg struct
emailAuthMsg.proof.emailNullifier in EmailAuthMsg struct
timestampCheckEnabled in EmailAuth
verifyEmailProof in Verifier

To avoid misuse of a Boolean literal, ensure it aligns with intended behavior and is well documented.

-> Because we are using these values and functions in a clear usage, it can be ignored.

Low

1. transfer and send calls are no longer considered best practice -> Fixed
2. Pragma statement spans several minor versions of solidity -> can be ignored, it's relevant in Groth16Verifier.sol
3. Unsafe abi encoding -> Fixed
4. Different pragma directives are used -> can be ignored, it's relevant in Groth16Verifier.sol
5. Floating pragma -> can be ignored?
6. Missing docstrings -> can be ignored, we only have that in public or external functions
7. Missing return statement -> can be ignored, it's relevant in Groth16Verifier.sol
8. External call failure checks are missing -> can be ignored, it's relevant in Groth16Verifier.sol

[SoraSuegami]

Thank you for your security checks and updates!
Could you explain how each update (each modification to each contract) fix vulnerabilities?

[wshino]

@SoraSuegami

1. transfer and send calls are no longer considered best practice -> Fixed

https://eips.ethereum.org/EIPS/eip-1884
This is a response to the repricing of some of the opcodes in EIP-1884.
Existing transfers use a minimal amount of gas, but the default access to storage may take more than that gas, in which case the transfer may fail.
Address.sendValue avoids that problem.

3. Unsafe abi encoding -> Fixed

Because abi.encodeWithSelector is not typesafe, it does not check functions and the arguments used for them at compile time.
The abi.encodeCall will check if it is the correct argument type for the function at compile time.

https://solidity-by-example.org/abi-encode/