Consistently refer to RFC 7159

[Marcono1234]

Parts of Gson refer to RFC 4627 while other parts refer to RFC 7159 (which updates 4627).
It would be good to consistently refer to RFC 7159. This requires:

• Updating the links
• Updating any section references / quotations of the RFC

[marcus-h]

It might make sense to directly refer to RFC 8259. For the JsonReader class, this is already part of
PR #1609 (which adds a new parsing mode to ensure that only JSON text is accepted that
complies to RFC 8259).

[Marcono1234]

Has been fixed by #2437, which consistently refers to RFC 8259.