Change MUST to SHOULD

[Zarquan]

Change MUST to SHOULD in this block of text.

https://github.com/ivoa-std/ExecutionPlanner/blob/978495a7df8c7d3dad688400a7240ca9f4db76c0/ExecutionPlanner.tex#L1408-L1411

The client SHOULD, because we won't guarantee what will happen if you use a different one, but it won't necessarily break anything if you do. We should add "the results are undefined if the client uses a different authentication method".