Drop node v18

[mcollina]

No description provided.

[mcollina]

cc @trivikr

[KhafraDev]

I haven't seen any reason to drop v18. Nothing in fetch or other web specs needs it gone, and it's in maintenance mode for another 5 months. Is there any reason other than precedence? If precedence is the main concern, there were genuine issues with previous versions of node that required us to drop them - I can't think of any similar issues in v18.

[trivikr]

@KhafraDev I'd provided estimates on how long unidici maintainers have to support Node.js 18.x in #3386 (comment) after it's end-of-life.
In summary, it needs to be supported for at least 6 months under the assumption that undici@v8 is released in Dec 2025.

Matteo also provided how dropping Node.js 18.x will ease maintenance of undici@v7 in #3386 (comment)

[trivikr]

As per #3386 (comment) on tracking issue, @KhafraDev is -0 in dropping support for Node.js 18.x in undici@v7

[trivikr]

LGTM, but this needs approval from a maintainer.