Use HttpHeaders#headerSet where appropriate

[spencergibb]

See spring-projects/spring-framework#33823

[KENNYSOFT]

https://github.com/spring-projects/spring-framework/blob/v6.1.15/spring-web/src/main/java/org/springframework/http/HttpHeaders.java#L1873-L1887

/**
 * Return a view of the headers as an entry {@code Set} of key-list pairs.
 * Both {@link Iterator#remove()} and {@link java.util.Map.Entry#setValue}
 * are supported and mutate the headers.
 * <p>This collection is guaranteed to contain one entry per header name
 * even if the backing structure stores multiple casing variants of names,
 * at the cost of first copying the names into a case-insensitive set for
 * filtering the iteration.
 * @return a {@code Set} view that iterates over all headers in a
 * case-insensitive manner
 * @since 6.1.15
 */
public Set<Entry<String, List<String>>> headerSet() {
	return new CaseInsensitiveEntrySet(this.headers);
}

So, Spring Cloud Gateway 4.1.6 (in Spring Cloud 2023.0.4) requires Spring 6.1.15, available in Spring Boot 3.2.12, 3.3.6, or 3.4.0.