stages/user_write: ensure user data is json-serializable #18690

Job	Run time
lint-golint	37s
test-unittest	2m 57s
ci-outpost-mark	1s
build-container (proxy)	3m 30s
build-binary (proxy, linux, amd64)	1m 51s
build-container (ldap)	2m 13s
build-binary (proxy, linux, arm64)	1m 58s
build-container (radius)	2m 9s
build-binary (ldap, linux, amd64)	1m 48s
build-container (rac)	7m 9s
build-binary (ldap, linux, arm64)	1m 49s
build-binary (radius, linux, amd64)	1m 36s
build-binary (radius, linux, arm64)	1m 49s
build-binary (rac, linux, amd64)	1m 34s
build-binary (rac, linux, arm64)	1m 50s
	32m 51s

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