GenAI: Rename prompt and completion tokens attributes to input and output

[lmolkova]

We're moved away from prompt/completion terminology on GenAI metrics to accommodate non-chat operations (like embeddings) - see #955 (comment) for the context.

But we didn't rename corresponding attributes on spans.

Changes

Rename:

• gen_ai.usage.completion_tokens to gen_ai.usage.output_tokens
• gen_ai.usage.prompt_tokens to gen_ai.usage.input_tokens

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• schema-next.yaml updated with changes to existing conventions.

[lmolkova]

/cc @open-telemetry/semconv-llm-approvers

[drewby]

This looks good. I just had one suggestion around making the descriptions consistent.