cannot compile fhir R4 schema

**What version of Ajv are you using? Does the issue happen if you use the latest version?**
8.6.2
**Ajv options object**

```javascript

```

**JSON Schema**

```
https://www.hl7.org/fhir/fhir.schema.json.zip
```

**Sample data**

```json

```

**Your code**

```
ajv compile -s schema/fhir.schema.json -o validate_schema.js
```

**Validation result, data AFTER validation, error messages**

```
schema schema/fhir.schema.json is invalid
error: strict mode: unknown keyword: "discriminator"
```

**What results did you expect?**

**Are you going to resolve the issue?**