Document 3.8.0 release notes in changelog

[Copilot]

This PR updates the release documentation so 3.8.0 is ready to ship. It adds a new dated changelog entry summarizing the two user-impacting changes included in this release.

• Release notes

  • Added ## [3.8.0] - 2026-09-21 under [Unreleased] in changelog.md.

• Changed

  • Documented the cbauth upgrade to ^7.0.0 for thread-safe authentication startup behavior.

• Fixed

  • Documented the Strict-Transport-Security header formatting correction (removal of the extra ": " in the emitted value).

## [3.8.0] - 2026-09-21

### Changed
- Upgraded `cbauth` to `^7.0.0` to support thread-safe authentication startup.

### Fixed
- Corrected the `Strict-Transport-Security` header formatting by removing the extra `": "` in the rendered header value.

[lmajano]

[3.8.0] - 2026-09-21

Don’t add this. This is automatic

[lmajano]

@copilot ## [3.8.0] - 2026-09-21

don’t add this

[Copilot]

@copilot ## [3.8.0] - 2026-09-21

don’t add this

Removed that manual version heading in commit 8e44c25, and kept the notes under [Unreleased] only.