Update CHANGELOG.md in preparation for branching v3.1 (#8791)

Update CHANGELOG.md in preparation for branching v3.1.

CHANGELOG.md

@@ -8,12 +8,19 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 and follows a [Backwards Compatibility Policy](https://docs.anza.xyz/backwards-compatibility)
 
 Release channels have their own copy of this changelog:
-* [edge - v3.1](#edge-channel)
-* [beta - v3.0](https://github.com/anza-xyz/agave/blob/v3.0/CHANGELOG.md)
-* [stable - v2.3](https://github.com/anza-xyz/agave/blob/v2.3/CHANGELOG.md)
+* [edge - v4.0](#edge-channel)
+* [beta - v3.1](https://github.com/anza-xyz/agave/blob/v3.1/CHANGELOG.md)
+* [stable - v3.0](https://github.com/anza-xyz/agave/blob/v3.0/CHANGELOG.md)
 
 <a name="edge-channel"></a>
-## 3.1.0—Unreleased
+## 4.0.0-Unreleased
+### RPC
+#### Breaking
+#### Changes
+### Validator
+#### Breaking
+
+## 3.1.0
 ### RPC
 #### Breaking
 * A signature verification failure in `simulateTransaction()` or the preflight stage of `sendTransaction()` will now be attached to the simulation result's `err` property as `TransactionError::SignatureFailure` instead of being thrown as a JSON RPC API error (-32003). Applications that already guard against JSON RPC exceptions should expect signature verification errors to appear on the simulation result instead. Applications that already handle the materialization of `TransactionErrors` on simulation results can now expect to receive errors of type `TransactionError::SignatureFailure` at those verification sites.