feat(ingress): migrate nginx annotation proxy_set_header to HAProxy

[x10an14-nav]

This preserves request-header behavior for applications relying on nginx.ingress.kubernetes.io/configuration-snippet during ingress migration.

HAProxy header names are case-insensitive, including the req.fhdr(...) lookup we use.
Sources:
- HAProxy docs, HTTP request headers:
  https://docs.haproxy.org/2.8/configuration.html#1.2.2

Quote:
> Contrary to a common misconception, header names are not case-sensitive

Also:
> Internally, all header names are normalized to lower case so that HTTP/1.x and HTTP/2 use the exact same representation
- HAProxy docs, hdr(<name>):
  https://docs.haproxy.org/2.8/configuration.html#4.2-balance

Quote:
> the header name in parenthesis is not case sensitive
- HAProxy source confirms req.fhdr(...) uses http_get_htx_fhdr(...), which calls http_find_header(...).
- http_find_header(...) uses isteqi(n, name) for exact header-name matching.
- isteqi(...) is HAProxy’s case-insensitive string equality function.

Conclusion:
- %[req.fhdr(x-client-ip)]
- %[req.fhdr(X-Client-IP)]
- %[req.fhdr(X-CLIENT-IP)]
all target the same request header name in HAProxy.