Change `generate_text` to `text_completion`

[alexander-alderman-webb]

The devdocs use text_completion: https://develop.sentry.dev/sdk/telemetry/traces/modules/ai-agents/
OTel usees text_completion: https://opentelemetry.io/docs/specs/semconv/gen-ai/gen-ai-spans/

• Remove GEN_AI_GENERATE_TEXT
• Add GEN_AI_TEXT_COMPLETION = "text_completion"
• Replace occurrences of the "generate_text" in span descriptions, gen_ai.operation.name attributes, and op with text_completion

[linear-code]

PY-2268