SecurityEvaluationContextExtension Signature

SecurityEvaluationContextExtension.getRootObject() should have a signature
with a return type of SecurityExpressionRoot. See
EvaluationContextExtension javadoc
https://github.com/spring-projects/spring-data-commons/blob/eed7658ce54dfc3960c28cd90f551fdd27b4adba/src/main/java/org/springframework/data/spel/spi/EvaluationContextExtension.java#L61-L64

This enhancement will allow for Spring Data to make certain types of
optimizations.

Fixes gh-7891

Author: rwinch

data/src/main/java/org/springframework/security/data/repository/query/SecurityEvaluationContextExtension.java

@@ -103,7 +103,7 @@ public String getExtensionId() {
 	}
 
 	@Override
-	public Object getRootObject() {
+	public SecurityExpressionRoot getRootObject() {
 		Authentication authentication = getAuthentication();
 		return new SecurityExpressionRoot(authentication) {
 		};