feat(next): introduce the openapi generated client in the next package

[ikadix]

Now available in github.com/krystal/go-katapult/next is a complete client for the katapult API.
This is generated from the openapi spec.
For now this includes a docker image and a shell script for the build command.

Note

This bumps the following things :

• Go Version is now minimum v1.20
• Golangci-lint is now using v1.56
• checkout action is now v4
• setup-go action is now v5
• golangci-lint action is now in use with v4
• codeclimate action is now v5

goconst has been disabled in test files.
a few changes have been made across the project to remove usage of io/ioutil

Closes #150

[jimehk]

Couple of minor tweaks to the CI workflows are needed, and I have a question about the bundled OpenAPI schema. Overall looks good and promising though :)

[jimehk]

Do we have any script and/or Make target that can update this file from the production URL?

[ikadix]

Added make targets for getting the schema from the url and for running the generate script

[codeclimate]

Code Climate has analyzed commit 4efa020 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (90% is the threshold).

This pull request will bring the total coverage in the repository to 91.0% (0.0% change).

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