Add changelog for 0.21.0 release #5833

Job	Run time
build_windows (3.10)	38m 56s
build_windows (3.9)	39m 40s
build_linux (3.13)	35m 19s
build_linux (3.10)	34m 20s
build_windows (3.12)	38m 20s
build_windows (3.11)	38m 58s
build_windows (3.13)	38m 17s
build_linux (3.11)	35m 17s
build_linux (3.9)	33m 26s
build_linux (3.12)	34m 45s
test_examples_linux (3.11, false, ubuntu-22.04)	8m 57s
array-api-conformity (3.10, false, ubuntu-22.04)	5m 28s
test_linux (3.9, false, ubuntu-22.04)	11m 54s
test_linux (3.11, false, ubuntu-22.04)	10m 50s
test_linux (3.12, false, ubuntu-22.04)	10m 58s
test_linux (3.10, false, ubuntu-22.04)	10m 36s
test_linux (3.13, false, ubuntu-22.04)	11m 29s
test_windows (3.10, false, windows-latest)	14m 57s
test_windows (3.12, false, windows-latest)	16m 56s
test_windows (3.9, false, windows-latest)	16m 3s
test_windows (3.13, false, windows-latest)	13m 22s
test_windows (3.11, false, windows-latest)	13m 36s
upload_linux	0s
upload_windows	0s
Clean up anaconda packages	0s
	8h 32m 24s

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