chore: allow php 8 as a platform requirement (#111)

Author: marianogoldman

composer.json

@@ -17,7 +17,7 @@
         }
     ],
     "require": {
-        "php": "^7.2",
+        "php": "^7.4|^8.0",
         "bitwasp/bitcoin": "^1.0",
         "kodekeep/binary": "^1.0",
         "kodekeep/bytebuffer": "^1.0",