[FrameworkBundle] Update the xsd to match the actual session configuration

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Update the FrameworkBundle xsd to match the actual session configuration

Author: Anthony MARTIN

Resources/config/schema/symfony-1.0.xsd

@@ -120,6 +120,7 @@
         <xsd:attribute name="gc-probability" type="xsd:string" />
         <xsd:attribute name="use-strict-mode" type="xsd:boolean" />
         <xsd:attribute name="save-path" type="xsd:string" />
+        <xsd:attribute name="metadata-update-threshold" type="xsd:nonNegativeInteger" />
     </xsd:complexType>
 
     <xsd:complexType name="request">