go.mod: update bart dep to v0.26.1 (#18659)

bart has gained a bunch of purported performance and usability
improvements since the current version we are using (0.18.0,
from 1y ago)

Updates tailscale/corp#36982

Signed-off-by: Amal Bansode <amal@tailscale.com>

go.mod

@@ -37,7 +37,7 @@ require (
 	github.com/fogleman/gg v1.3.0
 	github.com/frankban/quicktest v1.14.6
 	github.com/fxamacker/cbor/v2 v2.9.0
-	github.com/gaissmai/bart v0.18.0
+	github.com/gaissmai/bart v0.26.1
 	github.com/go-json-experiment/json v0.0.0-20250813024750-ebf49471dced
 	github.com/go-logr/zapr v1.3.0
 	github.com/go-ole/go-ole v1.3.0