Consolidate beta changelog entries into v16.2.0.beta.8 (#2050)

Consolidates the scattered changelog entries from beta releases (beta.4
through beta.8) into a single cohesive release entry for v16.2.0.beta.8.
This improves documentation organization and makes it easier to track
what's included in this beta version. Updates version comparison links
in the changelog footer.

🎯 Changes:
- Reorganizes changelog sections for better readability
- Consolidates duplicate entries and improves grouping
- Updates version comparison links for v16.2.0.beta.8

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## Summary by CodeRabbit

* **New Features**
  * Added Doctor Checks for async Loading Strategy
  * Added `--verbose` option for bin/dev command
  * Introduced Shakapacker Template Configuration

* **Improved**
  * Concurrent Streaming Performance enhancements
  * Generator Configuration Modernization with new defaults
  * Error messaging improvements

* **Documentation**
  * Updated release notes and migration guidance for v16.2.0.beta.8

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Co-authored-by: Claude <[email protected]>

Author: justin808