Describe the bug

ParamsSerializerOptions contains type definitions that are unnecessary from an implementation standpoint.

https://github.com/DigitalBrainJS/axios/blob/92f0758429541b05c614dc25bc712379d8a41dbd/lib/helpers/buildURL.js#L51-L57

Under the current situation where documentation is not well maintained, mistakes and misunderstandings can occur with the following argument types.
Check the following examples.

axios.create({
  paramsSerializer: {
    encode: value => value
    serialize: value => value // Value not referenced at runtime
  }
})

So we should remove the pattern of receiving both functions by modifying the type.
Or you should update the documentation.

PR: #5770

To Reproduce

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Expected behavior

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