Update OpenAPIKit to v4

[iT0ny]

Context

Duplicate of #830. This PR implements the additional changes/fixes for issues surfaced in the original PR

Summary

• Move OpenAPIKit to 4.x
• Refresh handling of null values
• Fix tests

Details

• Bump OpenAPIKit version so we target 4.x, and raise the Yams requirement to 5.1+
• Update wording in testEmitsComplexOpenAPIParsingError and testSchemaWarningsForwardedToGeneratorDiagnostics (Tests/OpenAPIGeneratorCoreTests/Parser/Test_YamsParser.swift, Tests/OpenAPIGeneratorCoreTests/Translator/TypesTranslator/Test_translateSchemas.swift)
• Update checks for null values in Sources/_OpenAPIGeneratorCore/Translator/CommonTranslations/translateRawEnum.swift
• Update OpenAPI.Content.Encoding usage according to the OpenAPIKit v4 migration guide in Tests/OpenAPIGeneratorCoreTests/Translator/TypeAssignment/Test_TypeMatcher.swift

Testing

• ✅ swift test
• CI run

Fixes #821

[czechboy0]

Thank you @iT0ny, which of the changes were required when bumping the OpenAPIKit version, and which ones were nice to have?

[iT0ny]

Thank you @iT0ny, which of the changes were required when bumping the OpenAPIKit version, and which ones were nice to have?

All of them are required, tests mentioned in the PR message were failing. + there was an issue with nil checks after the version bump