Update pragma to 0.8.20 (#4489)

Co-authored-by: Francisco Giordano <fg@frang.io>
Co-authored-by: ernestognw <ernestognw@gmail.com>

scripts/generate/templates/Checkpoints.js

@@ -3,7 +3,7 @@ const { OPTS } = require('./Checkpoints.opts.js');
 
 // TEMPLATE
 const header = `\
-pragma solidity ^0.8.19;
+pragma solidity ^0.8.20;
 
 import {Math} from "../math/Math.sol";