apigw-dynamodb-terraform: Update AWS Provider to v6

[kakakakakku]

Issue #, if available:
N/A

Description of changes:
Hi😀 Thanks for the useful patterns!

To keep this pattern maintainable, I updated the AWS Provider to v6 and replaced deprecated HCL code. I also fixed incorrect instructions in the README.

Check

terraform apply completed successfully and works good.

$ curl -H 'x-api-key: xxx' -H 'Content-Type: application/json' --request POST 'https://kl1vikew8a.execute-api.us-east-1.amazonaws.com/v1/pets' --data-raw '{ "PetType": "dog", "PetName": "tito", "PetPrice": 250 }'
{}

$ curl -H 'x-api-key: xxx' --request GET 'https://kl1vikew8a.execute-api.us-east-1.amazonaws.com/v1/pets/dog' | jq .
{
  "pets": [
    {
      "id": "f3667862-28d8-44cb-b4b6-c876a8991df9",
      "PetType": "dog",
      "PetName": "tito",
      "PetPrice": "250"
    }
  ]
}

Thank you😀

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[kakakakakku]

hash_key - (Optional, Deprecated) Name of the hash key in the index; must be defined as an attribute in the resource. Mutually exclusive with key_schema. Use key_schema instead.
https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/dynamodb_table

[kakakakakku]

note: It deprecated from Provider v6

[kakakakakku]

note: The README file was incorrect, preventing the tests from running properly. I've fixed it👍