feat(osv): add Unbuntu severity type to proto (google#3584)

ossf/osv-schema#337 introduces Ubuntu severity
type to the schema and this PR adds this severity type to
`vulnerability.proto`.

Author: cuixq

gcp/api/v1/api_descriptor.pb

@@ -119,6 +119,7 @@ message Severity {
     CVSS_V3 = 1;
     CVSS_V2 = 2;
     CVSS_V4 = 3;
+    UBUNTU = 4;
   }
   // The type of this severity entry.
   Type type = 1;

osv/vulnerability.proto

@@ -274,6 +274,7 @@ class Severity(google.protobuf.message.Message):
         CVSS_V3: Severity._Type.ValueType  # 1
         CVSS_V2: Severity._Type.ValueType  # 2
         CVSS_V4: Severity._Type.ValueType  # 3
+        UBUNTU: Severity._Type.ValueType  # 4
 
     class Type(_Type, metaclass=_TypeEnumTypeWrapper):
         """Type of the severity."""
@@ -282,6 +283,7 @@ class Severity(google.protobuf.message.Message):
     CVSS_V3: Severity.Type.ValueType  # 1
     CVSS_V2: Severity.Type.ValueType  # 2
     CVSS_V4: Severity.Type.ValueType  # 3
+    UBUNTU: Severity.Type.ValueType  # 4
 
     TYPE_FIELD_NUMBER: builtins.int
     SCORE_FIELD_NUMBER: builtins.int