Add TODO comment for bridgetree dependency downgrading

Author: dmidem

zebra-chain/Cargo.toml

@@ -68,6 +68,7 @@ bitflags = "2.5.0"
 bitflags-serde-legacy = "0.1.1"
 blake2b_simd = "1.0.2"
 blake2s_simd = "1.0.2"
+# TODO: Revert to "0.6.0" (or appropriate version) when the ZSA orchard fork is updated.
 bridgetree = "0.4.0"
 bs58 = { version = "0.5.1", features = ["check"] }
 byteorder = "1.5.0"
@@ -178,7 +179,3 @@ required-features = ["bench"]
 [[bench]]
 name = "redpallas"
 harness = false
-
-# FIXME: remove this and all zcash_unstable usage in the code after updating librustzcash
-[lints.rust]
-unexpected_cfgs = { level = "warn", check-cfg = ['cfg(zcash_unstable, values("nu6"))'] }