fix: correctly parse and validate forwarded client IPs and remove unmaintained chi-middleware/proxy (#39190)

Replace the unmaintained `chi-middleware/proxy` with an in-tree handler
that detects client IPs correctly behind common reverse proxies.

1. Handle repeated header lines and comma-separated proxy hops.
2. Accept and normalize IPs, including `ip:port`.
3. Keep the client port from the header value or `X-Real-Port` instead
of logging port 0.
4. Prefer the last `X-Real-IP` line.
5. Keep the proxy's address when the selected value is invalid.
6. Read `Forwarded` only when `X-Forwarded-For` is absent.

Docs PR: https://gitea.com/gitea/docs/pulls/560
Fixes: https://github.com/go-gitea/gitea/issues/35682
Fixes: https://github.com/go-gitea/gitea/issues/36814
Co-authored-by: bircni <bircni@icloud.com>

custom/conf/app.example.ini

@@ -471,7 +471,8 @@ INTERNAL_TOKEN =
 ;REVERSE_PROXY_AUTHENTICATION_EMAIL = X-WEBAUTH-EMAIL
 ;REVERSE_PROXY_AUTHENTICATION_FULL_NAME = X-WEBAUTH-FULLNAME
 ;;
-;; Interpret X-Forwarded-For header or the X-Real-IP header and set this as the remote IP for the request
+;; Interpret the X-Real-IP, X-Forwarded-For or Forwarded header, in that order, and set this as the remote IP for the request.
+;; The client port is taken from the header value or, with X-Real-IP, from X-Real-Port.
 ;REVERSE_PROXY_LIMIT = 1
 ;;
 ;; List of IP addresses and networks separated by comma of trusted proxy servers. Use `*` to trust all.