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phrase, deprecating the outdated `CHECKPOINT` enum value for 103.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-test/src/main/java/org/springframework/test/web/servlet/result/StatusResultMatchers.java

@@ -145,8 +145,18 @@ public class StatusResultMatchers {
 
 	/**
 	 * Assert the response status code is {@code HttpStatus.CHECKPOINT} (103).
+	 * @see #isEarlyHints()
+	 * @deprecated in favor of {@link #isEarlyHints()}
 	 */
+	@Deprecated(since = "6.0")
 	public ResultMatcher isCheckpoint() {
+		return isEarlyHints();
+	}
+
+	/**
+	 * Assert the response status code is {@code HttpStatus.EARLY_HINTS} (103).
+	 */
+	public ResultMatcher isEarlyHints() {
 		return matcher(HttpStatus.valueOf(103));
 	}