[FLINK-39619][docs] Add release notes for Flink 2.3#28123

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lihaosky wants to merge 3 commits intoapache:masterfrom

confluentinc:flink-2.3-release-note

Commits on May 6, 2026

[FLINK-39619][docs] Add release notes for Flink 2.3
lihaosky
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[FLINK-39619][docs] Restrict release notes to FLIP-derived content
lihaosky
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[FLINK-39619][docs] Enrich release notes with details from FLIP umbrella tickets
lihaosky
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