cmd/tailscaled: disable state encryption / attestation by default (#18336)

TPM-based features have been incredibly painful due to the heterogeneous
devices in the wild, and many situations in which the TPM "changes" (is
reset or replaced). All of this leads to a lot of customer issues.

We hoped to iron out all the kinks and get all users to benefit from
state encryption and hardware attestation without manually opting in,
but the long tail of kinks is just too long.

This change disables TPM-based features on Windows and Linux by default.
Node state should get auto-decrypted on update, and old attestation keys
will be removed.

There's also tailscaled-on-macOS, but it won't have a TPM or Keychain
bindings anyway.

Updates #18302
Updates #15830

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

cmd/tailscaled/tailscaled.go

@@ -910,13 +910,8 @@ func handleTPMFlags() {
 			log.Fatalf("--hardware-attestation is not supported on this platform or in this build of tailscaled")
 		}
 	case !args.hardwareAttestation.set:
-		policyHWAttestation, _ := policyclient.Get().GetBoolean(pkey.HardwareAttestation, feature.HardwareAttestationAvailable())
-		if !policyHWAttestation {
-			break
-		}
-		if feature.TPMAvailable() {
-			args.hardwareAttestation.v = true
-		}
+		policyHWAttestation, _ := policyclient.Get().GetBoolean(pkey.HardwareAttestation, false)
+		args.hardwareAttestation.v = policyHWAttestation
 	}
 
 	switch {
@@ -927,13 +922,8 @@ func handleTPMFlags() {
 			log.Fatal(err)
 		}
 	case !args.encryptState.set:
-		policyEncrypt, _ := policyclient.Get().GetBoolean(pkey.EncryptState, feature.TPMAvailable())
-		if !policyEncrypt {
-			// Default disabled, no need to validate.
-			return
-		}
-		// Default enabled if available.
-		if err := canEncryptState(); err == nil {
+		policyEncrypt, _ := policyclient.Get().GetBoolean(pkey.EncryptState, false)
+		if err := canEncryptState(); policyEncrypt && err == nil {
 			args.encryptState.v = true
 		}
 	}