Use 0.8.0 for echidna exercises

Author: gustavo-grieco

program-analysis/echidna/example/magic.sol

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: AGPL-3.0
-pragma solidity ^0.5.0;
+pragma solidity ^0.8.0;
 
 contract C {
     bool value_found = false;

program-analysis/echidna/exercises/Exercise-2.md

@@ -80,7 +80,7 @@ import "./token.sol";
 
 /// @dev Run the template with
 ///      ```
-///      solc-select use 0.5.0
+///      solc-select use 0.8.0
 ///      echidna program-analysis/echidna/exercises/exercise2/template.sol
 ///      ```
 contract TestToken is Token {

program-analysis/echidna/exercises/Exercise-3.md

@@ -104,7 +104,7 @@ import "./mintable.sol";
 
 /// @dev Run the template with
 ///      ```
-///      solc-select use 0.5.0
+///      solc-select use 0.8.0
 ///      echidna program-analysis/echidna/exercises/exercise3/template.sol --contract TestToken
 ///      ```
 contract TestToken is MintableToken {