chore: bump gunicorn version to `~21` (#1199) #1088

Job	Run time
build	1m 34s
test-country-template	30s
build-conda	0s
test-core	36s
lint-files	51s
test-extension-template	22s
check-version	19s
check-for-functional-changes	17s
deploy	45s
publish-to-conda	11m 2s
test-on-windows	5m 59s
	22m 15s

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