GitHub: add mongo version in template

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>

Author: Watson1978

.github/ISSUE_TEMPLATE/bug_report.yaml

@@ -41,6 +41,7 @@ body:
         - Fluentd version:
         - TD Agent version:
         - fluent-plugin-mongo version:
+        - mongo version:
         - Operating system:
         - Kernel version:
       render: markdown