Legacy import API requires "version" field

[jportner]

Kibana version: 7.9 and 7.10

Describe the bug: Prior to Kibana 7.9, the legacy import API (/api/kibana/dashboards/import) could be used without a "version" field in the body. When #69474 was released it appeared to have unintentionally marked the "version" field as required in the API route (due to changing from the Joi package to @kbn/config-schema). Joi defaults to optional but @kbn/config-schema defaults to required.

Steps to reproduce:

1. Start Kibana and Elasticsearch
2. Run the following curl command:

curl -XPOST -u elastic:changeme -H 'kbn-xsrf: true' -H "Content-type: application/json" -d '{
"objects": [
     {
        "attributes": {
            "timeFieldName": "@timestamp",
            "title": "some-title"
        },
        "id": "some-id",
        "type": "index-pattern"
     }
 ]
}' 'http://localhost:5601/api/kibana/dashboards/import'

and observe the error:

{"statusCode":400,"error":"Bad Request","message":"[request body.version]: expected value of type [string] but got [undefined]"

3. Run the following curl command with any version string added:

curl -XPOST -u elastic:changeme -H 'kbn-xsrf: true' -H "Content-type: application/json" -d '{
"version": "1",
"objects": [
     {
        "attributes": {
            "timeFieldName": "@timestamp",
            "title": "some-title"
        },
        "id": "some-id",
        "type": "index-pattern"
     }
 ]
}' 'http://localhost:5601/api/kibana/dashboards/import'

and observe that the command completed successfully.

Expected behavior: This API should not have added a required field in a minor release.

Any additional context: Viewing the code, it appears that this "version" field isn't even used anymore. It should be marked as optional.

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)