Stabilize exception semantic conventions.

[jsuereth]

Marks existing exception semconv as stable.

• These have been defacto stable for years now, with little changes.
• Any changes we want to make going forward needs to be non-breaking.
  • e.g. we may add a representation of stacktraces that would complement pure strings.
  • New Languages are free to add more stacktrace representations

If we're all agreed, I'll update the changelog.

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• CHANGELOG.md updated for non-trivial changes.
• schema-next.yaml updated with changes to existing conventions.