This avoids putting "DisablementSecrets" in the JSON output from
`tailscale lock log`, which is potentially scary to somebody who doesn't
understand the distinction.
AUMs are stored and transmitted in CBOR-encoded format, which uses an
integer rather than a string key, so this doesn't break already-created
TKAs.
Fixes#19189
Change-Id: I15b4e81a7cef724a450bafcfa0b938da223c78c9
Signed-off-by: Alex Chan <alexc@tailscale.com>