chore: migrate to ESLint v9 with flat config

[lotmek]

Description

• Fixes #309
• Remove ESLint Legacy config
• Add new flat config with the same rules as previous version
• Fix linting issues that were introduced with latest version of typescript-eslint:
  • no-unsafe-argument
  • no-floating-promises

Resources

• Migrate to v9.x
• Configuration Migration Guide

[kamilmysliwiec]

ESLint v9 requires node v20 so let's wait with this until the next major release

[ghost]

when we can have next major release ?

[neonbyte1]

11.0.0 has been published to npmjs, so I guess we can soon migrate to ESLint v9 @kamilmysliwiec?

[kamilmysliwiec]

https://github.com/nestjs/typescript-starter/commit/12d01ee39d400320431a46ccc2f0e8558a841daa