Release v5.1.0 (rc) (#5124)

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Hadrien Croubois <hadrien.croubois@gmail.com>
Co-authored-by: Ernesto García <ernestognw@gmail.com>

package.json

@@ -1,7 +1,7 @@
 {
   "name": "openzeppelin-solidity",
   "description": "Secure Smart Contract library for Solidity",
-  "version": "5.0.2",
+  "version": "5.1.0-rc.0",
   "private": true,
   "files": [
     "/contracts/**/*.sol",