fix(plugins): add realm to removed fields

[nowNick]

Summary

Basic-auth and key-auth added new field "realm" but it was not added to "removed_fields" which breaks backwards compat between new CPs and old DPs. Adding realm to removed fields fixes the issue.

Checklist

• [x] The Pull Request has tests
• [x] A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• [x] There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

KAG-4516