Upgrade solhint to 2.1.0 (#1781)

* Upgrade solhint to 2.1.0

* Use compiler-version ^0.5.0

.solhint.json

 {
-  "extends": "default",
+  "extends": "solhint:recommended",
   "rules": {
     "indent": ["error", 4],
-    "func-order": false,
-    "bracket-align": false,
-    "compiler-fixed": false,
-    "no-simple-event-func-name": false,
-    "separate-by-one-line-in-contract": false,
-    "two-lines-top-level-separator": false
+    "func-order": "off",
+    "bracket-align": "off",
+    "compiler-fixed": "off",
+    "no-simple-event-func-name": "off",
+    "separate-by-one-line-in-contract": "off",
+    "two-lines-top-level-separator": "off",
+    "mark-callable-contracts": "off",
+    "compiler-version": ["error", "^0.5.0"]
   }
 }

package.json

@@ -61,7 +61,7 @@
     "nodemon": "^1.19.0",
     "openzeppelin-docsite": "github:OpenZeppelin/openzeppelin-docsite#ee1df82d52ad3df4337b20392627975091f8d5b3",
     "openzeppelin-test-helpers": "^0.4",
-    "solhint": "^1.5.0",
+    "solhint": "2.1.0",
     "solidity-coverage": "github:rotcivegaf/solidity-coverage#5875f5b7bc74d447f3312c9c0e9fc7814b482477",
     "solidity-docgen": "github:OpenZeppelin/solidity-docgen#03f42b5e271b1e1c1d0b814b921ecdc086055255",
     "truffle": "^5.0.18"