[chore] Add link to migration guide in RPC README & CODEOWNERS

[kamphaus]

Changes

Following-up on PR comment #3224 (comment), add a link to the migration guide in the RPC README (similar to HTTP README).
Also adds the RPC migration guide to CODEOWNERS like it's done for other migration docs.

Important

Pull requests acceptance are subject to the triage process as described in Issue and PR Triage Management.
PRs that do not follow the guidance above, may be automatically rejected and closed.

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry. → NA
  • If your PR does not need a change log, start the PR title with [chore]
• Links to the prototypes or existing instrumentations (when adding or changing conventions) → NA