fix(VulnerabilityReference): Do not deserialize a lazy property

This is a fixup for 8b6fe4f which introduced the `severityRating`
property.

Signed-off-by: Sebastian Schuberth <sebastian@doubleopen.org>

model/src/main/kotlin/vulnerabilities/VulnerabilityReference.kt

@@ -19,6 +19,8 @@
 
 package org.ossreviewtoolkit.model.vulnerabilities
 
+import com.fasterxml.jackson.annotation.JsonIgnoreProperties
+
 import java.net.URI
 
 /**
@@ -30,6 +32,7 @@ import java.net.URI
  * with a list of references; each reference points to the source of the information and has some detailed information
  * provided by this source.
  */
+@JsonIgnoreProperties(value = ["severity_rating"], allowGetters = true)
 data class VulnerabilityReference(
     /**
      * The URI pointing to details of this vulnerability. This can also be used to derive the source of this