dnsdist: Properly override the HTTP Server header for DoH

[rgacogne]

Short description

h2o doesn't allow us to override the HTTP Server: header in the usual way but we can override the default value in the h2o_globalconf_t object.

Fixes #7894.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)