Deprecate Base64Utils in favor of Java's Base64

Originally, `Base64Utils` was providing base64 encoding and decoding
utilities, bridging to commons-codecs or Java 8, if available. Since
then, only the Java 8 variant remains and Spring Framework 6 requires
now Java 17. This utility class doesn't provide additional checks or
syntactic sugar over what's in Java already.

As a result, this commit deprecates this class in favor of `Base64` and
schedules the removal of this class completely.

Closes gh-28434

spring-core/src/main/java/org/springframework/util/Base64Utils.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -27,7 +27,9 @@ import java.util.Base64;
  * @author Gary Russell
  * @since 4.1
  * @see java.util.Base64
+ * @deprecated as of Spring Framework 6.0.5 in favor of {@link Base64}.
  */
+@Deprecated(since = "6.0.5", forRemoval = true)
 public abstract class Base64Utils {
 
 	/**