Use m.thread over m.in_reply_to

[germain-gg]

Following up on matrix-org/matrix-spec-proposals#3440

We want to move away from using m.in_reply_to to build threads, and instead use a new relation type m.thread.

• io.element.thread should be used in place of m.thread as relation type before the MSC has shipped
• Make sure that m.thread can be combined with m.in_reply_to to allow quote relies in threads
• Remove all support for io.element.in_thread and m.in_thread