Messaging semconv migration guide

[joaopgrassi]

Fixes #1183

Changes

Migration guide similar to the one done for HTTP, for when the messaging conventions become stable.

Merge requirement checklist

• [x] CONTRIBUTING.md guidelines followed.
• [ ] Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• [ ] schema-next.yaml updated with changes to existing conventions.