Move /admin/whois back to /_matrix namespace

[turt2live]

It's in the spec since forever: matrix-org/matrix-spec-proposals#198 / https://matrix.org/docs/spec/client_server/r0.4.0.html#get-matrix-client-r0-admin-whois-userid

Note: matrix-org/matrix-spec-proposals#1474 hints at the section possibly disappearing, but I can't remember what the decision was on this. https://github.com/matrix-org/matrix-doc/issues/1411 wants the "appropriate" APIs to be in the spec (through MSC).

let the arguments to (not) keep the endpoint in the spec begin

[richvdh]

the /client/r0 path should still work (for now)?

[turt2live]

For now, but not when develop goes mainline

[richvdh]

related: #5119

[richvdh]

@turt2live I think we agreed this wasn't an issue because the /r0 path isn't going away any time soon. Shout if I'm misremembering.