Remove undici dependency

[ejizba]

Background: Removing this was the plan all along when we created the new programming model. Part of the reason we chose undici for our http types was because it was being integrated into Node.js core at the time (with v18), but unfortunately it was still marked experimental in Node.js core even though the standalone package was marked stable. The support in Node.js core was just marked stable in Node.js v22 (see here) so we can now use the core module directly instead of the standalone package. Removing this by far has the biggest and best impact on cold start, as the undici standalone package has an annoyingly large number of files that is slowing things down.

Breaking change: Customers should see very little functional impact. The risk here is more around TypeScript builds getting broken due to slightly different request/response types coming from a slightly different place. The biggest breaking change is that this relies on Node.js v22 and our current minimum for v4 of the package is v18.

Branch to remove: ej/removeUndici