fix: pin zk-hardhat solidity to 0.8.28 (#8543)

Author: paulbalaji

solidity/zk-hardhat.config.cts

@@ -10,11 +10,13 @@ import { rootHardhatConfig } from './rootHardhatConfig.cjs';
  */
 module.exports = {
   ...rootHardhatConfig,
-  // Override solidity version — era-solidity only supports up to 0.8.30
+  // Override solidity version — zkvm-solc 0.8.30 crashes with EPIPE on CI
+  // when compiling the full contract set. Pin to 0.8.28 (>=0.8.24 required
+  // for transient storage opcodes used by TransientStorage.sol).
   // https://github.com/matter-labs/era-solidity/releases
   solidity: {
     ...rootHardhatConfig.solidity,
-    version: '0.8.30',
+    version: '0.8.28',
   },
   zksolc: {
     version: '1.5.12',