Per trigger delivery spec should not be a MUST

[zhongduo]

Describe the bug
In #4654 the Per trigger delivery spec is set as a MUST, this will break existing broker implementations that cannot support it.

Expected behavior
We should keep compatibility with brokers that cannot support per trigger delivery spec by changing it to SHOULD.

[zhongduo]

/cc @slinkydeveloper @vaikas @lionelvillard @grantr