feat: add /schema/openapi.yml to the available schema paths

[JacobCoffee]

Pull Request Checklist

• New code has 100% test coverage
• (If applicable) The prose documentation has been updated to reflect the changes introduced by this PR
• (If applicable) The reference documentation has been updated to reflect the changes introduced by this PR
• Pre-Commit Checks were ran and passed
• Tests were ran and passed

Description

• Updates all CI files to standardize on .yml (we had a mix before)
• Adds a .yml available path for generate_openapi_schema, its respective routes in litestar.openapi.controller.OpenAPIController.retrieve_schema_yaml, and accompanying tests.

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[cofin]

LGTM and no issue with having them both there.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

100.0% Coverage
0.0% Duplication

[github-actions]

Documentation preview will be available shortly at https://litestar-org.github.io/litestar-docs-preview/2411

[provinzkraut]

Just fyi: This rename breaks our PyPI pipeline. It needs the workflow filename hardcoded, so if you're changing this, you'll also need to change it there.