Remove engine requirement from package.json

[Aarbel]

@JustinBeckwith

i fixed this, because this package is used by other libraries that still need to run on older node versions.

Imposing node >=18 on an utilitary library like this one is maybe not an "open source community" good practice

cf https://github.com/JustinBeckwith/retry-axios/blob/main/package.json, here are examples of the package.json files of the dependencies used by retry-axios:

  "peerDependencies": {
    "axios": "*"
  },
  "devDependencies": {
    "@biomejs/biome": "^1.8.3",
    "@types/mocha": "^10.0.1",
    "@types/node": "^20.0.0",
    "@types/sinon": "^17.0.0",
    "axios": "^1.2.1",
    "c8": "^10.0.0",
    "js-green-licenses": "^4.0.0",
    "mocha": "^10.2.0",
    "nock": "^13.3.0",
    "semantic-release": "^24.0.0",
    "sinon": "^18.0.0",
    "typescript": "~5.5.0"
  },

• https://github.com/axios/axios/blob/v1.x/package.json
• https://github.com/mochajs/mocha/blob/main/package.json
• https://github.com/nock/nock/blob/main/package.json
• https://github.com/bcoe/c8/blob/main/package.json
• https://github.com/sinonjs/sinon/blob/main/package.json