Migrate CHANGELOG from specification over, keeping only SEMCONV sections.

[jsuereth]

No description provided.

[reyang]

This is partially resolved by #18 (do we need historical changes or we can point to the spec repo?).

[arminru]

@reyang I think either is fine. It would be a one-time effort to copy them over and would provide a nicer experience (one can ctrl+f in this one single file) but adding a line saying the following should also be enough:

Previously, the semantic conventions in this repository were part of open-telemetry/opentelemetry-specification, for which the changelog can be found here.

[jsuereth]

Our automation for schema_url basically requires historical changes. Effectively it uses version numbers in CHANGELOG.md to verify schema_url accounts for all of them.

See: https://github.com/open-telemetry/semantic-conventions/blob/main/internal/tools/schema_check.sh#L18