Prior to this commit, the GroovyBeanDefinitionReader claimed (via its
Javadoc) that it fully supported XML configuration files in addition to
its Groovy DSL; however, this was unfortunately inaccurate since XML
validation was disabled by default which led to certain features of XML
configuration not working. For example, it was impossible to define a
<qualifier> in an XML config file without specifying the 'type'
attribute (which has a default value defined in the spring-beans XSD).

This commit fixes this issue by ensuring that bean definitions in XML
resources are loaded with a "standard" XmlBeanDefinitionReader that is
created with default settings (i.e., with XML validation enabled). With
regard to backwards compatibility, bean definitions defined using the
Groovy DSL are still loaded with an XmlBeanDefinitionReader that has
XML validation disabled by default which is necessary for proper
parsing of the Groovy DSL.

Issue: SPR-12769