Turn on allow_variance_keywords for xplat/js/react-native-github/.github.flowconfig (#56931)

Summary:
Pull Request resolved: #56931

default to true version is not released yet. turning it on

Changelog: [Internal]

Reviewed By: gkz

Differential Revision: D106027737

fbshipit-source-id: f55ec41471a23208d803d2b984fcf62105fad89a

Author: marcoww6

.flowconfig

@@ -45,6 +45,7 @@ packages/react-native/flow/
 [options]
 enums=true
 experimental.pattern_matching=true
+experimental.allow_variance_keywords=true
 casting_syntax=both
 component_syntax=true