Remove support for application service authentication via query params

[clokep]

#13996 added support for MSC2832 to authenticate appservices via a header instead of query parameters. It didn't deprecate or remove the query params authentication for backwards compatibility.

This is unfortunate since passing those will end up in proxy logs, etc. #15317 intends to "deprecate" them and I'd like to disable them by default / remove them, but technically this would mean that Synapse is no longer spec compliant for spec versions < 1.4.

We have a few options for this:

• Don't care about the spec compliant issue and just remove support.
• Add a configuration flag (either in homeserver.yaml or an implementation-specific flag in the appservice config file) to enable support for legacy authentication.
• Drop support for Matrix < 1.4.

There's a few other ideas in #15317 too.

Note that #15317 declares the current status-quo until at least Synapse v1.88.0.

[erikjohnston]

• Add a configuration flag (either in homeserver.yaml or an implementation-specific flag in the appservice config file) to enable support for legacy authentication.

Let's go with this option, as a compromise.