NEXT-35968 - Fix OpenApi Schema validation

[akf-bw]

1. Why is this change necessary?

• Currently the github workflow for the OpenApi Schema validation fails because it's not setup correctly
• Also the action fails because a few api specs in the core a wrong

2. What does this change do, exactly?

• Changed 01-lint-openapi to run correctly & cache composer files
• Changed CalculatedPrice API Schema to correctly use oneOf with null
• Changed Product API Schema to correctly use oneOf with null
• Changed ReferencePrice API Schema to correctly use oneOf with null

3. Describe each step to reproduce the issue or behaviour.

• Run the OpenApi Schema validation github action

4. Please link to the relevant issues (if any).

• /

5. Checklist

• I have rebased my changes to remove merge conflicts
• I have written tests and verified that they fail without my change
• I have created a changelog file with all necessary information about my changes
• I have written or adjusted the documentation according to my changes
• This change has comments for package types, values, functions, and non-obvious lines of code
• I have read the contribution requirements and fulfil them.

[akf-bw]

@philipreinken I fixed the GitHub action to correctly run and changed it to match the structure of the other actions.

[shopware-github-importer]

Hello,

thank you for creating this pull request.
I have opened an issue on our Issue Tracker for you. See the issue link: https://issues.shopware.com/issues/NEXT-35968

Please use this issue to track the state of your pull request.

[BrocksiNet]

@mkucmus, can you please validate the changes made to OpenAPI Schema? Thx.