Bump ajv from 6.12.2 to 6.12.6 in /nodejs

Bumps [ajv](https://github.com/ajv-validator/ajv) from 6.12.2 to 6.12.6.
- [Release notes](https://github.com/ajv-validator/ajv/releases)
- [Commits](https://github.com/ajv-validator/ajv/compare/v6.12.2...v6.12.6)

---
updated-dependencies:
- dependency-name: ajv
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>

nodejs/package-lock.json

@@ -298,9 +298,9 @@
       }
     },
     "ajv": {
-      "version": "6.12.2",
-      "resolved": "https://registry.npmjs.org/ajv/-/ajv-6.12.2.tgz",
-      "integrity": "sha512-k+V+hzjm5q/Mr8ef/1Y9goCmlsK4I6Sm74teeyGvFk1XrOsbsKLjEdrvny42CZ+a8sXbk8KWpY/bDwS+FLL2UQ==",
+      "version": "6.12.6",
+      "resolved": "https://registry.npmjs.org/ajv/-/ajv-6.12.6.tgz",
+      "integrity": "sha512-j3fVLgvTo527anyYyJOGTYJbG+vnnQYvE0m5mmkc1TK+nxAppkCLMIL0aZ4dblVCNoGShhm+kzE4ZUykBoMg4g==",
       "dev": true,
       "requires": {
         "fast-deep-equal": "^3.1.1",