Fix support for CJS and add support for Mongoose 8 (and MongoDB 7)

[T-vK]

This PR fixes #453 and #454. I've tested it with CommonJS and ESM. Build, Test, Lint etc. are working fine too.

Notable changes

Regarding #453:

• Fixed support for Common JS. (express-restify-mongoose 8 broke support for CJS, meaning it could only be used with import (ESM) and not with require (CJS) anymore)
• Downgraded serialize-error to ^8.1.0 and dot-prop to ^6.0.0 in order to add support for CJS again.
• Removed ESM build (because CJS automatically supports ESM as well and swc is lacking in features to properly build a dual module)
• Had to switch to .mjs extension to allow tests to keep using import
• Fixed version of eslint-plugin-mocha to 10.3.0 because with 10.4.0 the minimum node version was increased to 18.
• Removed "type": "module" from the package.json (necessary to support CJS)
• Bumped ERM version to 9.0.0

Regarding #454:

• Uses Mongoose 8 by default now (adding support for MongoDB 7)
• Supports Mongoose 7+8 in addition to what was supported before
• Minimum required node version increased to 16 (requirement for Mongoose 8)
• Since Mongoose 7 and 8 dropped support for callbacks, I had to rewrite some of the code into the promise syntax.
• Mongoose 7+8 also deprecated and removed some functions which therefore had to be replaced. E.g. .findOneAndRemove() and .remove()...
• In one test I had to do add delete body.errors.customer.message; in order keep support for older Mongoose versions, which in my opinion doesn't have a negative impact.

[T-vK]

@florianholzapfel @Zertz I'd love to get this merged soon if possible. :)

[florianholzapfel]

thanks for the awesome work!