Remove `_experiments` replay options compat shim in v9

## Summary

The `_experiments.replaysSessionSampleRate` and `_experiments.replaysOnErrorSampleRate` options were deprecated in favor of the root-level `replaysSessionSampleRate` / `replaysOnErrorSampleRate` options. A compat shim in `packages/core/src/js/integrations/default.ts` copies the old values to the new location at init time.

The original comment said "Remove in the next major version (v7)", but this was missed in both v7 and v8. Updated to v9 in #5881.

## Action for v9

- Remove the compat shim in `default.ts` (lines 144-154)
- Remove the `_experiments.replaysSessionSampleRate` and `_experiments.replaysOnErrorSampleRate` type definitions from `options.ts`
- Consider also removing the `initialScope` option (deprecated since v5, replacement: `getGlobalScope()` / `getIsolationScope()`)

## References

- [SDK Deprecation Lifecycle Standard](https://develop.sentry.dev/sdk/)
- #5881