go.toolchain.rev: bump Tailscale Go version to 1.21.6

Updates tailscale/go#83

Signed-off-by: James Tucker <[email protected]>

go.toolchain.rev

@@ -1 +1 @@
-7592922bcb2af4e0b49bc67d7923fbdabe168dab
+ea90ced9ddc95c09aed7d9c59631aa978022c3ba