Add release notes for 2.21.x #22473

Job	Run time
Classify changes	4s
Ensure PR has a category label	2s
Bootstrap Pants, test Rust (macOS11-x86_64)	0s
Build wheels (Linux-ARM64)	0s
Ensure PR has release notes	1s
Bootstrap Pants, test Rust (Linux-ARM64)	0s
Bootstrap Pants, test and lint Rust (Linux-x86_64)	0s
Build wheels (macOS10-15-x86_64)	0s
Build wheels (macOS11-ARM64)	0s
Build wheels (Linux-x86_64)	0s
Test Python (macOS11-x86_64)	0s
Test Python (Linux-ARM64)	0s
Lint Python and Shell	0s
Test Python (Linux-x86_64) Shard 0/10	0s
Test Python (Linux-x86_64) Shard 1/10	0s
Test Python (Linux-x86_64) Shard 2/10	0s
Test Python (Linux-x86_64) Shard 3/10	0s
Test Python (Linux-x86_64) Shard 4/10	0s
Test Python (Linux-x86_64) Shard 5/10	0s
Test Python (Linux-x86_64) Shard 6/10	0s
Test Python (Linux-x86_64) Shard 7/10	0s
Test Python (Linux-x86_64) Shard 8/10	0s
Test Python (Linux-x86_64) Shard 9/10	0s
Set Merge OK	1s
Merge OK	1s
	9s

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