Bump version to 0.4.21 (#911) #460

Job	Run time
docker-build	3m 16s
plano-tools-tests	19s
native-smoke-test	4m 59s
validate-config	21s
pre-commit	2m 50s
test-prompt-gateway	2m 7s
security-scan	54s
test-model-alias-routing	3m 2s
e2e-demo-currency	1m 4s
test-responses-api-with-state	2m 5s
e2e-plano-tests (3.12)	47s
e2e-plano-tests (3.11)	43s
e2e-plano-tests (3.10)	42s
e2e-plano-tests (3.13)	45s
e2e-plano-tests (3.14)	45s
e2e-demo-preference	2m 46s
	27m 25s

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