Added unaffected_versions to CVE-2021-31799 (closes #836).

* Versions below 3.11.0 are not affected.
  GHSA-ggxm-pgc9-g7fp

Author: postmodern

gems/rdoc/CVE-2021-31799.yml

@@ -12,6 +12,8 @@ description: |
   run an arbitrary command execution against a user who attempts to run `rdoc`
   command.
 cvss_v3: 7.0
+unaffected_versions:
+  - "< 3.11.0"
 patched_versions:
   - "~> 6.1.2.1"
   - "~> 6.2.1.1"