Document breaking changes in changelog

[nikhilNava]

Updated changelog to reflect breaking changes in version 0.2.1.dev46.

[github-actions]

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[Copilot]

Pull request overview

Updates the observability-core package changelog to document breaking changes for the 0.2.1.dev46 development version.

Changes:

• Replaces the top-level ## [Unreleased] heading with ## [0.2.1.dev46].

[Copilot]

This package changelog no longer has an ## [Unreleased] section, which is inconsistent with the other library changelogs in this repo and makes it harder to keep tracking upcoming changes. Consider restoring ## [Unreleased] at the top and adding ## [0.2.1.dev46] as the next section (moving the breaking changes under the versioned heading), even if Unreleased is empty for now.

Suggested change

	## [Unreleased]