Remove `_experiments` replay options compat shim in v9

[antonis]

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
• fix(core): Fix stale deprecation references #5881

[linear-code]

RN-552