Support Undici v6

[ralbertazzi]

🚀 Feature Proposal

The library currently constrains undici to ^5.22.1. Undici v6 has been out since almost 2 months, @elastic/transport should support it. I don't think big breaking changes have been brought, expect that Node 16, which has reached EOL, is not supported anymore (PR).

Motivation

Keep up with security fixes, bug fixes, performance improvements, ...

[JoshMock]

This could also help solve our long-running MaxListenersExceededWarning issue, assuming that the next Undici release includes nodejs/undici#2823.

[JoshMock]

Waiting to tie up some loose ends on #85. If that merges soon, v8.14.0 of the client will be running on Undici v6.