mgr/dashboard: Simplify authentication protocol

mgr/dashboard: Simplify authentication protocol
By removing the dependency to PyJWT we also remove the dependency to the cryptographic library which
in the dashboard module will create a crash. In newer implementations of the library PyO3 is used to run
rust code in order to encrypt with Elliptic Curves. This is never used in the dashboard communication so
a much simpler implementation where we only use the hmac sha256 algorithm to create the signed JWT message
could be used.

Fixes: https://forum.proxmox.com/threads/ceph-warning-post-upgrade-to-v8.129371
Signed-off-by: Daniel Persson <mailto.woden@gmail.com>