Fix alias resolution error message in SimpleAliasRegistry

Prior to this commit, the alias resolution error message in
SimpleAliasRegistry was misleading.

When a resolution conflict is detected, the IllegalStateException
thrown by resolveAliases(...) now states that the resolved alias is
already registered for an `existingName` instead of the `registeredName`.

See gh-31353
Closes gh-32025

spring-core/src/main/java/org/springframework/core/SimpleAliasRegistry.java

@@ -172,7 +172,7 @@ public class SimpleAliasRegistry implements AliasRegistry {
 						throw new IllegalStateException(
 								"Cannot register resolved alias '" + resolvedAlias + "' (original: '" + alias +
 								"') for name '" + resolvedName + "': It is already registered for name '" +
-								registeredName + "'.");
+								existingName + "'.");
 					}
 					checkForAliasCircle(resolvedName, resolvedAlias);
 					this.aliasMap.remove(alias);