Consolidate v0.87.0-RC changelog entries into a single v0.87.0 section

Summary:
The 0.87 release notes were split across five release-candidate sections
(rc.0 through rc.4). Merge them into a single v0.87.0 section without changing
any entry text:

- Combine all five RC sections under a single `v0.87.0` heading
- Organize entries under the standard Breaking / Added / Changed / Deprecated /
  Removed / Fixed / Security categories, each with general / Android specific /
  iOS specific subsections
- Sort entries within every subsection by their bold prefix
- Triage the changelog generator's `Unknown` and `Android Unknown` buckets into
  real categories rather than dropping them: the `EventEmitter.cpp` data-race
  fix moves to `Fixed`, and the remaining automated Flow, Kotlin, i18n, and
  build-tooling entries move to `Changed`
- Drop empty subsection headings
- No entries were added, removed, or reworded

Changelog: [Internal]

Differential Revision: D115575927

Author: zeyap