Undo open specification of JSON compatibility

PkgRegistrator doesn't like this (JuliaRegistries/General#32561)

Author: vtjnash

Project.toml

@@ -11,7 +11,7 @@ UUIDs = "cf7118a7-6976-5b1a-9a39-7adc72f591a4"
 
 [compat]
 julia = "1"
-JSON  = "≥ 0.18"
+JSON  = "0.18, 0.19, 0.20, 0.21"
 
 [extras]
 Statistics = "10745b16-79ce-11e8-11f9-7d13ad32a3b2"