Introduce MapAccessor in SpEL and deprecate existing implementation

Prior to this commit, the MapAccessor for SpEL resided in the
org.springframework.context.expression package in the spring-context
module; however, it arguably should reside in the spring-expression
module so that it can be used whenever SpEL is used (without requiring
spring-context).

This commit therefore officially deprecates the MapAccessor in
spring-context for removal in favor of a new copy of the implementation
in the org.springframework.expression.spel.support package in the
spring-expression module.

Closes gh-35537

spring-context/src/test/java/org/springframework/context/expression/MapAccessorTests.java

@@ -36,6 +36,7 @@ import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
  * @author Andy Clement
  * @author Sam Brannen
  */
+@SuppressWarnings("removal")
 class MapAccessorTests {
 
 	private final StandardEvaluationContext context = new StandardEvaluationContext();