docs: add greater pubsub org as owners of managedkafka directory

Description

Fixes #

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[ ] I have followed Sample Format Guide
[ ] pom.xml parent set to latest shared-configuration
[ ] Appropriate changes to README are included in PR
[ ] These samples need a new API enabled in testing projects to pass (let us know which ones)
[ ] These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
[ ] Tests pass: mvn clean verify required
[ ] Lint passes: mvn -P lint checkstyle:check required
[ ] Static Analysis: mvn -P lint clean compile pmd:cpd-check spotbugs:check advisory only
[ ] This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
[ ] This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
[x] Please merge this PR for me once it is approved