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/key_test.go

@@ -73,8 +73,8 @@ func TestNLPrivate(t *testing.T) {
 	// authority.
 	k := Key{Kind: Key25519, Public: pub.Verifier(), Votes: 1}
 	_, aum, err := Create(ChonkMem(), State{
-		Keys:               []Key{k},
-		DisablementSecrets: [][]byte{bytes.Repeat([]byte{1}, 32)},
+		Keys:              []Key{k},
+		DisablementValues: [][]byte{bytes.Repeat([]byte{1}, 32)},
 	}, p)
 	if err != nil {
 		t.Fatalf("Create() failed: %v", err)