Update DataSourceBuilder aliases

This commit adds a `user` alias for the `username` property which permits
the use of `OracleDataSource`.

Closes gh-6124, gh-6027, gh-6125

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jdbc/DataSourceBuilder.java

@@ -83,7 +83,10 @@ public class DataSourceBuilder {
 
 	private void bind(DataSource result) {
 		MutablePropertyValues properties = new MutablePropertyValues(this.properties);
-		new RelaxedDataBinder(result).withAlias("url", "jdbcUrl").bind(properties);
+		new RelaxedDataBinder(result)
+				.withAlias("url", "jdbcUrl")
+				.withAlias("username", "user")
+				.bind(properties);
 	}
 
 	public DataSourceBuilder type(Class<? extends DataSource> type) {