APIF-1506: Update api/v3/openapi.yaml

[linouk23]

Some context

https://confluent.slack.com/archives/C012SG54VM1/p1631870864138900

What

This PR:

• Removes v3 substring from method's operationIds. Why? This change only affects SDKs and the idea is we want to change from ClusterV3Api.getKafkaV3Cluster() -> ClusterV3Api.getKafkaCluster() (operationId is effectively a method name).
• Adds (v3) suffix to a tag name. Why? We wanna avoid having both ClusterApi and ClusterV3Api in generated client.go:

Testing

Run against OpenAPI validator: https://apitools.dev/swagger-parser/online/ ✅

[linouk23]

➜  kafka-rest git:(update-spec) swagger-cli validate api/v3/openapi.yaml
api/v3/openapi.yaml is valid
➜  kafka-rest git:(update-spec) openapi-spec-validator api/v3/openapi.yaml
OK
➜  kafka-rest git:(update-spec) openapi-examples-validator api/v3/openapi.yaml
Validating examples
Schemas with examples found: 43
Examples without schema found: 0
Total examples found: 443

No errors found.

[AndrewJSchofield]

Thanks for the PR. I have made the same changes under a much wider review of the OpenAPI spec so this PR is no longer required.