standardize C-like MIT copyright headers throughout fbsource

Summary:
`/*` is the standard throughout open source code. For example, Firefox uses single /*: https://hg.mozilla.org/mozilla-central/file/21d22b2f541258d3d1cf96c7ba5ad73e96e616b5/gfx/ipc/CompositorWidgetVsyncObserver.cpp#l3

In addition, Rust considers `/**` to be a doc comment (similar to Javadoc) and having such a comment at the beginning of the file causes `rustc` to barf.

Note that some JavaScript tooling requires `/**`. This is OK since JavaScript files were not covered by the linter in the first place, but it would be good to have that tooling fixed too.

Reviewed By: zertosh

Differential Revision: D15640366

fbshipit-source-id: b4ed4599071516364d6109720750d6a43304c089

iOS/Plugins/FlipperKitUserDefaultsPlugin/FKUserDefaultsPlugin.h

@@ -1,3 +1,9 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the LICENSE
+ * file in the root directory of this source tree.
+ */
 //
 //  FKUserDefaultsPlugin.h
 //  Sample

iOS/Plugins/FlipperKitUserDefaultsPlugin/FKUserDefaultsSwizzleUtility.h

@@ -1,3 +1,9 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the LICENSE
+ * file in the root directory of this source tree.
+ */
 //
 //  FKUserDefaultsSwizzleUtility.h
 //  FlipperKit