Made ajv an explicit dependency

refs https://github.com/TryGhost/Toolbox/issues/314

- The `ajv` package is widely used and specifying it as a dependency has gone unnoticed. Not having it in depenency list can lead to pretty hairy problems once the rest of dependencies bump up the version.
- We are 2 majors behind at the moment, but at least we can work on upgrading it conciously without a surprise bump at some point in the future

Author: naz

packages/admin-api-schema/package.json

@@ -26,6 +26,7 @@
   },
   "dependencies": {
     "@tryghost/errors": "^1.0.0",
+    "ajv": "^6.12.6",
     "lodash": "^4.17.11"
   }
 }

yarn.lock

@@ -1240,7 +1240,7 @@ acorn@^8.5.0, acorn@^8.7.1:
   resolved "https://registry.yarnpkg.com/acorn/-/acorn-8.7.1.tgz#0197122c843d1bf6d0a5e83220a788f278f63c30"
   integrity sha512-Xx54uLJQZ19lKygFXOWsscKUbsBZW0CPykPhVQdhIeIwrbPmJzqeASDInc8nKBnp/JT6igTs82qPXz069H8I/A==
 
-ajv@^6.10.0, ajv@^6.12.4:
+ajv@^6.10.0, ajv@^6.12.4, ajv@^6.12.6:
   version "6.12.6"
   resolved "https://registry.yarnpkg.com/ajv/-/ajv-6.12.6.tgz#baf5a62e802b07d977034586f8c3baf5adf26df4"
   integrity sha512-j3fVLgvTo527anyYyJOGTYJbG+vnnQYvE0m5mmkc1TK+nxAppkCLMIL0aZ4dblVCNoGShhm+kzE4ZUykBoMg4g==