BinaryReader: Use cached default encoding instance (#8017)

BinaryReader: Use cached default encoding instance (#8017)

A new instance of UTF8Encoding used to be created every time
BinaryReader.ctor(Stream) was called, which created an instance of
UTF8Encoding that has no preamble. However, BinaryReader does not use
the preamble at all, so it doesn't matter if the encoding has a preamble
or not. Thus, the cached Encoding.UTF8 instance can be used (which has a
preamble).