Update Firebase C++/Unity SDK version number to 6.0.0. (plus guids)

PiperOrigin-RevId: 246402653

Author: jonsimantov

cpp_sdk_version.json

@@ -1,5 +1,5 @@
 {
-  "released": "5.7.0",
-  "stable": "5.7.0",
-  "head": "5.7.0"
+  "released": "6.0.0",
+  "stable": "6.0.0",
+  "head": "6.0.0"
 }