ref(core): Use user attribute constants from conventions (#24974)

Uses `USER_ID`, `USER_EMAIL`, `USER_IP_ADDRESS`, and `USER_NAME` from
`@sentry/conventions/attributes`. Deprecates the existing core constants
while preserving their public exports and identical attribute keys;
emitted values and data-collection behavior remain unchanged.
`USER_NAME` still maps the existing username to `user.name`.

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

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