Remove `stdout` option as default and implement base logging

[austinvalle]

This PR:

• Removes stdout as an option for outputting the IR, related PR for removing docs in common: https://github.com/hashicorp/terraform-docs-common-internal/pull/190
• Upgrades to go 1.21 for log/slog
• Upgrades to latest of codegen-spec and adds version field
• Adds the basic setup for structured logging
• Other small refactoring + package docs

Notes

For OpenApi Specs that have circular references (like my example below with stripe's OAS), you'll now receive a warning like below:

time=2023-09-26T16:08:35.006-04:00 level=WARN msg="circular reference found in OpenAPI spec" circular_ref="payment_intent -> customer -> invoice_setting_customer_setting -> payment_method -> payment_method_card -> payment_method_card_generated_card -> setup_attempt -> api_errors -> payment_intent"

This may result in a later error, but not always, which is why it's not considered a "fail" use-case 😄

[austinvalle]

Will have another PR coming up with more error handling, but the general idea behind the logging will be more descriptive errors to help identify where in the OAS the problem is. Example for an int/string multi-type:

https://github.com/kubernetes/kubernetes/blob/bf1fa0c6691bb3bc6c5c03f900c53ed609e580a4/api/openapi-spec/v3/apis__apps__v1_openapi.json#L879

time=2023-09-28T15:54:35.502-04:00 level=WARN msg="skipping resource schema mapping" resource=deployment_v1 err="[integer, string] - unsupported multi-type schema, attribute cannot be created" oas_property=spec.strategy.rollingUpdate.maxSurge oas_line_number=879

[austinvalle]

Not sure if it'll be an issue, but I bumped to 1.21 to use the stable log/slog. It's not a hard requirement for the logging so I can swap it and downgrade later if it's a problem 👍🏻

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