chore(deps): bump node to v18

[rflechtner]

re https://github.com/KILTprotocol/ticket/issues/3081

Require at least node v18 (maintenance), CI uses node v20 (current). Testing against v16 has moved to optional.
https://nodejs.org/en/about/previous-releases

How to test:

CI should not break.

Checklist:

• I have verified that the code works
• I have verified that the code is easy to understand
  • If not, I have left a well-balanced amount of inline comments
• I have left the code in a better state
• I have documented the changes (where applicable)
  • Either PR or Ticket to update the Docs
  • Link the PR/Ticket here

[ntn-x2]

Very good! I assume this is not a breaking change for downstream users? They would only get a warning if using a version < 18, right? Will this be released after SDK v1 is out?

[rflechtner]

Very good! I assume this is not a breaking change for downstream users? They would only get a warning if using a version < 18, right? Will this be released after SDK v1 is out?

Depending on their config the package manager may error, but they should be able to disable that.
And yes, id say this gets released with v1, otherwise we'd have to backport (which we may have to do eventually anyway though)