ref(core): Use HTTP method constant from conventions (#24977)

Uses `HTTP_REQUEST_METHOD` from `@sentry/conventions/attributes`.
Deprecates the existing core constant while preserving its public export
and identical `http.request.method` key; emitted values, method
normalization, defaults, and the GraphQL fallback to `http.method`
remain unchanged.

Part of #24961 / [JS-3868](https://linear.app/getsentry/issue/JS-3868).

Co-authored-by: GPT-6 <codex@openai.com>