[9.3] CODEOWNERS: remove gieselaar (#253766)

[kibanamachine]

Backport

This will backport the following commits from main to 9.3:

• CODEOWNERS: remove gieselaar (#253766)

Questions ?

Please refer to the Backport tool documentation

[elasticmachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 8691d2e

Failed CI Steps

• FTR Configs #73

Test Failures

• [job] [logs] FTR Configs #73 / Entity Analytics - Privilege Monitoring @ess @serverless @skipInServerlessMKI Entity Privilege Monitoring Engine Plain Index Sync Plain index sync should update a user when it was already added by the API

Metrics [docs]

✅ unchanged

cc @tylersmalley