Reassign flags no longer owned by Pipeline Execution

What does this MR do and why?

grouppipeline authoring owns #30098 (closed), which is blocking the removal of the ci_yaml_size_limit feature flag.

grouptesting owns #285597 (closed), which is blocking the removal of the s3_multithreaded_uploads feature flag.

See gitlab-org/quality/triage-reports#5474 (comment 749608453) and gitlab-org/quality/triage-reports#5474 (comment 751668362)

MR acceptance checklist

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