ref(core): Use cache attribute constants from conventions (#24973)

Uses `CACHE_HIT`, `CACHE_KEY`, and `CACHE_ITEM_SIZE` from
`@sentry/conventions/attributes`. Deprecates the existing core constants
while preserving their public exports and identical attribute keys;
emitted values and cache behavior remain unchanged.

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

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