Add semconv-1.31.0 changes to openai instrumentation

[SrdjanLL]

What does this pull request do?

• Use gen_ai.output.type instead of deprecated gen_ai.openai.request.response_format attribute.
• Use json instead of json_schema|json_object for gen_ai.output.type attribute value, as the enum options for the new attribute changed to be more generic (see here)
• Update tests to reflect the changes above.

Related issues

• Change in the semantic conventions: Introduce generic gen_ai.output.type instead of gen_ai.openai.request.response_format open-telemetry/semantic-conventions#1757

Closes #52

[xrmx]

LGTM, while you are at it could you please bump the schema url in src/opentelemetry/instrumentation/openai/__init__.py to 1.31? Thanks