Deprecate com.google.gwt.dev.util.Util for removal

[niloc132]

This has a TODO indicating that at least stream functions should be replaced with guava. Other IO functions should also move to Guava or java.nio, and xml functions have no need to exist at all. This reduces the class to just a few calls, and can be moved to somewhere more appropriate in the codebase, and the class deleted.

There are other TODOs in calls to methods in this class that can be cleaned up by this work as well.

Replacing usage of this class will help with a few classes of issues

• Masking IO errors by throwing NPE instead
• Buffering output that need not be buffered (such as reading into a bytearray, or writing a bytearray/string to a file in one step).

Fully completing this work suggests raising the java release level to 11.

See also #10011.