ref(core): Use exclusive time constant from conventions (#24971)

Uses `SENTRY_EXCLUSIVE_TIME` from `@sentry/conventions/attributes` in
span serialization, transaction conversion, and web-vital spans.
Deprecates the existing core constant while preserving its public export
and the identical `sentry.exclusive_time` key; calculations, values, and
serialization behavior remain unchanged.

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

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