Update Neutron version running in CI to v4.1.0 (#4133)

* Update Nix flake to use branch with Neutron v4.1.0

* Add changelog entry

* Set Neutron as chain with dynamic fees in tests

.changelog/unreleased/improvements/ibc-integration-test/4122-update-neutron-to-v4.md

@@ -0,0 +1,2 @@
+- Update the version of Neutron running the integration tests in the CI from `v3.0.5`
+  to `v4.1.0` ([\#4122](https://github.com/informalsystems/hermes/issues/4122))

tools/test-framework/src/chain/chain_type.rs

@@ -103,7 +103,9 @@ impl FromStr for ChainType {
 
     fn from_str(s: &str) -> Result<Self, Self::Err> {
         match s {
-            name if name.contains("gaiad") => Ok(ChainType::Cosmos { dynamic_fee: true }),
+            name if name.contains("gaiad") || name.contains("neutrond") => {
+                Ok(ChainType::Cosmos { dynamic_fee: true })
+            }
             name if name.contains("evmosd") => Ok(ChainType::Evmos),
             name if name.contains("injectived") => Ok(ChainType::Injective),
             name if name.contains("provenanced") => Ok(ChainType::Provenance),