Add "what's new" in 2.20 #22016

Job	Run time
Classify changes	3s
Ensure PR has a category label	2s
Bootstrap Pants, test Rust (Linux-ARM64)	1s
Build wheels (Linux-ARM64)	1s
Bootstrap Pants, test and lint Rust (Linux-x86_64)	1s
Bootstrap Pants, test Rust (macOS11-x86_64)	1s
Build wheels (macOS10-15-x86_64)	1s
Build wheels (Linux-x86_64)	1s
Build wheels (macOS11-ARM64)	1s
Lint Python and Shell	1s
Test Python (Linux-ARM64)	1s
Test Python (Linux-x86_64) Shard 0/10	1s
Test Python (Linux-x86_64) Shard 1/10	1s
Test Python (Linux-x86_64) Shard 2/10	1s
Test Python (Linux-x86_64) Shard 3/10	1s
Test Python (Linux-x86_64) Shard 4/10	1s
Test Python (Linux-x86_64) Shard 5/10	1s
Test Python (Linux-x86_64) Shard 6/10	1s
Test Python (Linux-x86_64) Shard 7/10	1s
Test Python (Linux-x86_64) Shard 8/10	1s
Test Python (Linux-x86_64) Shard 9/10	1s
Test Python (macOS11-x86_64)	1s
Set Merge OK	1s
Merge OK	1s
	27s

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