Downgrade hashbrown to meet MSRV

`hashbrown` depends on `ahash` which depends on `once_cell`. Sadly,
in matklad/once_cell#201 the `once_cell`
maintainer decided they didn't want to do the work of having an
MSRV policy for `once_cell`, making `ahash`, and thus `hashbrown`
require the latest compiler. I've reached out to `ahash` to suggest
they drop the dependency (as they could trivially work around not
having it), but until then we simply downgrade `hashbrown`.

`rust-bitcoin` also requires an older `hashbrown` so we're actually
reducing our total `no-std` code here anyway.

lightning/Cargo.toml

@@ -40,7 +40,7 @@ default = ["std", "grind_signatures"]
 [dependencies]
 bitcoin = { version = "0.29.0", default-features = false, features = ["secp-recovery"] }
 
-hashbrown = { version = "0.11", optional = true }
+hashbrown = { version = "0.8", optional = true }
 hex = { version = "0.4", optional = true }
 regex = { version = "1.5.6", optional = true }
 backtrace = { version = "0.3", optional = true }