Replace use of the <tt> HTML tag in Javadoc

The <tt> HTML tag was used in HTML 4 to define teletype text. It is not
supported in HTML5, and its use is discouraged.

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/tt

Closes gh-28819

spring-context/src/main/java/org/springframework/cache/Cache.java

@@ -24,7 +24,7 @@ import org.springframework.lang.Nullable;
  * Interface that defines common cache operations.
  *
  * <b>Note:</b> Due to the generic use of caching, it is recommended that
- * implementations allow storage of <tt>null</tt> values (for example to
+ * implementations allow storage of {@code null} values (for example to
  * cache methods that return {@code null}).
  *
  * @author Costin Leau