Update accessing config to use non-depricated methods

[ZenHarbinger]

Accessing config key '[auditLog]' through dot notation is deprecated, and it will be removed in a future release. Use 'config.getProperty(key, targetClass)' instead.

Update the ConfigObject accessor to be as follows:

    static ConfigObject getAuditConfig() {
        def grailsConfig = getApplication().config
        if (grailsConfig.containsProperty('auditLog')) {
            log.error "Your auditLog configuration settings use the old prefix 'auditLog' but must now use 'grails.plugin.auditLog'"
        }
        grailsConfig.getProperty('grails.plugin.auditLog', ConfigObject.class)
    }

Based on the spring security plugin:

https://github.com/grails/grails-spring-security-core/blob/8786567e45ed96b6ba891b611cf4cae26a79d30f/plugin/src/main/groovy/grails/plugin/springsecurity/ReflectionUtils.groovy#L114