Deprecate HttpStatus 103 CHECKPOINT in favor of new EARLY_HINTS (#29816)

This commit takes rfc8297 into account and introduces a newer code 103
HttpStatus value which uses `Early Hints` as the more correct reason
phrase, deprecating the outdated `CHECKPOINT` enum value for 103.

Additionally:
  - `HttpStatus.valueOf(103)` will return the new enum value
  - `HttpStatusCode#isSameCodeAs(HttpStatusCode)` is introduced to ease
  comparison of deprecated enums vs their newer counterparts (or any
  instance of a more generic `HttpStatusCode`) by comparing the integer
  `value()`
  - `HttpStatusTests` covers the new deprecation as well as the three
  previously deprecated codes, including a check with the above new
  method to ensure they have comparable integer values

Supersedes and Closes gh-27960

spring-web/src/main/java/org/springframework/http/HttpStatus.java

@@ -50,11 +50,18 @@ public enum HttpStatus implements HttpStatusCode {
 	 * @see <a href="https://tools.ietf.org/html/rfc2518#section-10.1">WebDAV</a>
 	 */
 	PROCESSING(102, Series.INFORMATIONAL, "Processing"),
+	/**
+	 * {code 103 Early Hints}.
+	 * @see <a href="https://tools.ietf.org/html/rfc8297">An HTTP Status Code for Indicating Hints</a>
+	 */
+	EARLY_HINTS(103, Series.INFORMATIONAL, "Early Hints"),
 	/**
 	 * {@code 103 Checkpoint}.
 	 * @see <a href="https://code.google.com/p/gears/wiki/ResumableHttpRequestsProposal">A proposal for supporting
 	 * resumable POST/PUT HTTP requests in HTTP/1.0</a>
+	 * @deprecated in favor of {@link #EARLY_HINTS} which will be returned from {@code HttpStatus.valueOf(103)}
 	 */
+	@Deprecated(since = "6.0")
 	CHECKPOINT(103, Series.INFORMATIONAL, "Checkpoint"),
 
 	// 2xx Success

spring-web/src/main/java/org/springframework/http/HttpStatusCode.java

@@ -76,6 +76,16 @@ public sealed interface HttpStatusCode extends Serializable permits DefaultHttpS
 	 */
 	boolean isError();
 
+	/**
+	 * Whether this {@code HttpStatusCode} shares the same integer {@link #value() value} as the other status code.
+	 * <p>Useful for comparisons that take deprecated aliases into account or compare arbitrary implementations
+	 * of {@code HttpStatusCode} (e.g. in place of {@link HttpStatus#equals(Object) HttpStatus enum equality}).
+	 * @param other the other {@code HttpStatusCode} to compare
+	 * @return true if the two {@code HttpStatusCode} share the same integer {@code value()}, false otherwise
+	 */
+	default boolean isSameCodeAs(HttpStatusCode other) {
+		return value() == other.value();
+	}
 
 	/**
 	 * Return an {@code HttpStatusCode} object for the given integer value.