ci: migrate unreleased changelog entries to towncrier fragments (#4588)

* migrate unreleased changelog entries to towncrier fragments

Convert existing entries from ## Unreleased sections into towncrier
fragment files, completing the migration started in #4573.

Root CHANGELOG.md: 25 entries migrated to .changelog/
propagator-aws-xray: 2 entries migrated
resource-detector-azure: 2 entries migrated
sdk-extension-aws: 2 entries migrated
opamp-client: 0 entries (section was empty)

The ## Unreleased sections are removed since towncrier now manages
changelog generation from fragment files at release time.

GenAI package changelogs are not migrated as those packages are
being moved out of this repo.

Assisted-by: Claude Opus 4.6

* fix fragment PR number for sdk-extension-aws

3366.added → 3761.added (3366 was the issue, 3761 is the PR)

Per review from @xrmx.

Assisted-by: Claude Opus 4.6

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Co-authored-by: Emídio Neto <9735060+emdneto@users.noreply.github.com>