tka: refer consistently to "DisablementValues"

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>

tka/sync_test.go

@@ -340,8 +340,8 @@ func TestSyncSimpleE2E(t *testing.T) {
         G1.template = genesis
     `,
 		optTemplate("genesis", AUM{MessageKind: AUMCheckpoint, State: &State{
-			Keys:               []Key{key},
-			DisablementSecrets: [][]byte{DisablementKDF([]byte{1, 2, 3})},
+			Keys:              []Key{key},
+			DisablementValues: [][]byte{DisablementKDF([]byte{1, 2, 3})},
 		}}),
 		optKey("key", key, priv),
 		optSignAllUsing("key"))