Upgrade ajv to v7

[SchoolGuy]

You cannot use the 2019-09 or newer schemas due to ajv supporting them only in v7 or later.

See here for context: ajv-validator/ajv#1432

I can reproduce this in my CI here: cobbler/libcobblersignatures#65

[SchoolGuy]

Actually, I am even getting the following error also with other versions?

Schema snippet:

{
  "$schema": "https://json-schema.org/draft-07/schema",
  ...
}

...
✗ /github/workspace/docs/schema_v1.json
Error - no schema with key or ref "https://json-schema.org/draft-07/schema"
...

[SchoolGuy]

Switching to

{
  "$schema": "http://json-schema.org/draft-07/schema",
  ...
}

fixed the issue for draft 7 of JSON Schema. Still an upgrade to v7 of ajv is in order IMHO.