Dropping readable-stream polyfill

[benmccann]

It looks like the CI tests Node 18.17+, so I guess that might be the minimum version supported by this library. Once that's raised to Node 18.19+ it would be nice to drop the readable-stream dependency. I'm guessing that would be a change in supported Node versions and would be considered a breaking change, so might have to wait until the next major.

[kibertoad]

Node 18 goes EOL on 2025-04-30, so personally I don't see a big problem with bumping minimum version to Node 20 in the nearby future (in a semver major, of course).
fastify has already done that.

as a bonus, node:test could be used instead of tap then

[mcollina]

I'm +1 on removing, send a PR.