message: contracts-governance: make the governance token immutable
  Slight improvement to the `MintManager`. Making
  the reference to the `governanceToken` an immutable
  is just better than placing it in storage.
diff: @@ -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
  