fractal-api: Change the well-known request to be a GET.

The well-known requests for homeserver discovery was a POST request
while the spec[1] states it should be a GET request.

[1] https://matrix.org/docs/spec/client_server/latest#get-well-known-matrix-client
2 jobs for poljar/well-known-get in 28 minutes and 26 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  .Pre
passed #592124
flatpak

00:27:25

 
  Check
passed #592125
check

00:01:00