chore: downgrade nodejs to 18

[weboko]

Problem

Some of the consumers of the library are using NodeJS mobile which has support only for nodejs@18.

Solution

Downgrade NodeJS requirement in js-waku temporarily.
We need to revert #2110

[weboko]

Important for the next release #2135

[danisharora099]

With experiments on PR #2173, downgrading to NodeJS18 will involve polyfilling some libraries with related dependencies that don't exist in Node18, which might also involve downgrading some js-waku dependencies that are not straightforward to polyfill.

The decision between polyfilling vs waiting for NodeJS mobile to upgrade to Node22 is a decision to make. Almost 40% of the upgrade on NodeJS mobile to Node22 has been achieved, and it might be better to hold off upgrading js-waku until the above PR is merged on NodeJS Mobile (ETA is end of October/2 weeks)

Assuming the library consumer in this context is @mattgle, what do you think?

cc @weboko

[weboko]

Closing as we agreed with Zy0n not to proceed with that and I believe @mattgle is on the same page.