resource/aws_api_gateway_account: Properly support unregistration

[gdavison]

Description

An aws_api_gateway_account can be unregistered by clearing the cloudwatch_role_arn. Do that and add sweeper.

Output from Acceptance Testing

% make testacc PKG=apigateway TESTS='TestAccAPIGateway_serial/Account'

--- PASS: TestAccAPIGateway_serial (178.47s)
    --- PASS: TestAccAPIGateway_serial/Account (178.47s)
        --- PASS: TestAccAPIGateway_serial/Account/basic (178.47s)

[github-actions[bot]]

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[github-actions[bot]]

This functionality has been released in v5.75.1 of the Terraform AWS Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!