nixos/mautrix-facebook: set verification levels

These are now required otherwise startup fails with:

> TypeError: 'NoneType' object is not subscriptable

The chosen levels are stricter than default but don't require unsupported signing or DB editing so seem like a reasonable high bar for now. It is easy for users to lower the levels so it is better to be stricter by default.

Default levels: 0ce0588725/mautrix_facebook/example-config.yaml (L247-L263)

nixos/modules/services/matrix/mautrix-facebook.nix

@@ -44,6 +44,12 @@ in {
             encryption = {
               allow = true;
               default = true;
+
+              verification_levels = {
+                receive = "cross-signed-tofu";
+                send = "cross-signed-tofu";
+                share = "cross-signed-tofu";
+              };
             };
             username_template = "facebook_{userid}";
           };