fix Open ID with Azure as provider

fix Open ID with Azure as provider

Azure doesn't accept `Transfer-Encoding: chunked` on their token endpoint,
but with the switch to ureq we always send requests with this set.

Fix by switching to `Content-Length` in the header instead. ureq only
sets `Transfer-Encoding: chunked` when the body length is not known
beforehand, which is the case when using `send`. See
https://docs.rs/ureq/2.4.0/ureq/index.html#content-length-and-transfer-encoding

See https://forum.proxmox.com/threads/openid-401-with-azure-ad.105892/
for the issue.

Signed-off-by: Mira Limbeck <m.limbeck@proxmox.com>