Consolidate changelog to keep only rc6 prerelease (#2533)

### Summary

This PR cleans up `CHANGELOG.md` so `16.4.0.rc.6` is the only non-final
release section remaining. It removes older non-final sections
(`16.4.0.rc.5`, `16.4.0.rc.3`, and `8.0.0-beta.*`) and updates compare
links to point from `16.3.0` to `16.4.0.rc.6`.

### Pull Request checklist

- [x] ~Add/update test to cover these changes~
- [x] ~Update documentation~
- [x] Update CHANGELOG file

### Other Information

`bundle exec rubocop` was run and failed due pre-existing offenses in
unrelated files; this PR only modifies `CHANGELOG.md`.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Documentation-only changes confined to `CHANGELOG.md` with no impact
on runtime behavior.
> 
> **Overview**
> Updates `CHANGELOG.md` to keep only a single active prerelease section
(`16.4.0.rc.6`), moving the relevant fixes (including the streaming
SSR/RSC fix) under that version and removing older prerelease/beta
sections.
> 
> Cleans up historical noise by deleting legacy `8.0.0-beta.*` entries
and updates the compare links at the bottom to point `unreleased` at
`v16.4.0.rc.6...master` (and adds the new `16.4.0.rc.6` compare link).
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
6185a69. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Bug Fixes**
* Improved streaming SSR reliability and payload handling to prevent
hangs and ensure proper error visibility.
* Ensured consistent instrumentation initialization for reliable
performance monitoring.
* Restored RSC support for SWC-based projects and fixed RSC generator
layout wiring for fresh installs.
* Fixed unsafe string interpolation in generated JS to prevent breakage.
* Made precompile hook detection and self-guarding more robust to avoid
duplicate runs in multi-process/HMR setups.

* **Changelog**
  * Updated release notes and anchors to rc.6.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: justin808