Update dreamfactory/df-core requirement from ~0.20.0 to ~0.21.0

Updates the requirements on [dreamfactory/df-core](https://github.com/dreamfactorysoftware/df-core) to permit the latest version.
- [Release notes](https://github.com/dreamfactorysoftware/df-core/releases)
- [Changelog](https://github.com/dreamfactorysoftware/df-core/blob/master/CHANGELOG.md)
- [Commits](dreamfactorysoftware/df-core@0.20.0...0.21.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Author: dependabot-preview[bot]

composer.json

@@ -29,7 +29,7 @@
   "minimum-stability": "dev",
   "prefer-stable": true,
   "require":     {
-    "dreamfactory/df-core":   "~0.20.0",
+    "dreamfactory/df-core":   "~0.21.0",
     "dreamfactory/df-system": "~0.4.0"
   },
   "autoload":    {