contracts-governance: make the governance token immutable (#3566)

Slight improvement to the `MintManager`. Making
the reference to the `governanceToken` an immutable
is just better than placing it in storage.
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

packages/contracts-governance/contracts/MintManager.sol

@@ -15,7 +15,7 @@ contract MintManager is Ownable {
     /**
      * @notice The GovernanceToken that the MintManager can mint tokens
      */
-    GovernanceToken public governanceToken;
+    GovernanceToken public immutable governanceToken;
 
     /**
      * @notice The amount of tokens that can be minted per year. The value is a fixed