BUG/MINOR: fix regsub backreference in RequestRedirect ReplacePrefixMatch

HAProxy's regsub converter does not support backreferences in the
substitution string. The generated expression

  %[path,regsub(^/old(/.*)?\$,/new\1)]

caused a parse error: 'missing arguments (got 1/2)' because \1 is not
a valid substitution in regsub.

regsub replaces only the matched portion of the string and leaves the
remainder intact, so no capture group is needed. Use the simpler form:

  %[path,regsub(^/old,/new)]

Author: ivanmatmati

.aspell.yml

@@ -109,3 +109,6 @@ allowed:
   - httpfilters
   - allowlist
   - inttest
+  - regsub
+  - backreference
+  - backreferences

k8s/gate/haproxy/filters/http_filters.go

@@ -384,7 +384,7 @@ func requestRedirectRules(f *gatewayv1.HTTPRequestRedirectFilter, matchPrefix st
 		}
 		escapedPrefix := regexEscapePath(matchPrefix)
 		return models.HTTPRequestRules{
-			redirect(fmt.Sprintf("%s%%[path,regsub(^%s(/.*)?$,%s\\1)]",
+			redirect(fmt.Sprintf("%s%%[path,regsub(^%s,%s)]",
 				pfx, escapedPrefix, replacement), "", ""),
 		}
 	}