make setters in GovernorSettings virtual

(cherry picked from commit 6e5bf052112b1ab72efafc1e9c20d71f15478d3e)

contracts/governance/extensions/GovernorSettings.sol

@@ -58,7 +58,7 @@ abstract contract GovernorSettings is Governor {
      *
      * Emits a {VotingDelaySet} event.
      */
-    function setVotingDelay(uint256 newVotingDelay) public onlyGovernance {
+    function setVotingDelay(uint256 newVotingDelay) public virtual onlyGovernance {
         _setVotingDelay(newVotingDelay);
     }
 
@@ -67,7 +67,7 @@ abstract contract GovernorSettings is Governor {
      *
      * Emits a {VotingPeriodSet} event.
      */
-    function setVotingPeriod(uint256 newVotingPeriod) public onlyGovernance {
+    function setVotingPeriod(uint256 newVotingPeriod) public virtual onlyGovernance {
         _setVotingPeriod(newVotingPeriod);
     }
 
@@ -76,7 +76,7 @@ abstract contract GovernorSettings is Governor {
      *
      * Emits a {ProposalThresholdSet} event.
      */
-    function setProposalThreshold(uint256 newProposalThreshold) public onlyGovernance {
+    function setProposalThreshold(uint256 newProposalThreshold) public virtual onlyGovernance {
         _setProposalThreshold(newProposalThreshold);
     }