httpserver: use own HTTP status codes

Summary:
Before, macros defined in <event2/http.h> have been used
for some HTTP status codes.
<event2/http.h> is included implicitly and the usage
of its status code macros is inconsistent with the majority
HTTP response implementations in this file.

Now, the HTTPStatusCode enum from <rpc/protocol.h> is
consistently used for all HTTP response implementations.

This is a backport of Core PR18168

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8779