feat: make telemetry off by default (#4281)
## Summary
Closes #3940
## Changes
- **Behavior:** `scarf_analytics()` sends the ping only when
`UNSTRUCTURED_TELEMETRY_ENABLED=true` (or `1`). Opt-out env vars
`DO_NOT_TRACK` and `SCARF_NO_ANALYTICS` are still respected and take
precedence.
- **Docs:** README Analytics section and logger comment updated to
describe the new default and opt-in/opt-out.
- **Tests:** New `DescribeScarfAnalytics` tests for default off, opt-in
(`true`/`1`), and opt-out overriding opt-in.
- **Changelog:** Entry under 0.21.13.
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Co-authored-by: Lawrence Elitzer (LoLo) <lawrence@unstructured.io>
