Deprecate OIDC Authorization code grant support and usage

The initial OIDC implementation in the openEO python client was focused on the Authorization code grant (the flow that works with redirect urls and such).
Getting this working in a python client context is far from trivial in practice because of the networking aspects of the redirect url.

Now that the Device Authorization Grant (aka device flow) is getting more popular and more widely supported, nobody is probably going to use the Authorization code grant.

Possible action points

• current documentation is currently is quite focused on Authorization Code Grant (in terms of priority and documentation volume). The device auth grant should get more focus instead
• remove implementation parts related to Auth Code Grant, which should make the OIDC implementation easier to maintain

Start by locating the Python client's OIDC implementation and documentation for the Authorization Code Grant, then compare them with the Device Authorization Grant flow. Done means the device flow receives the primary documentation focus and obsolete Authorization Code Grant implementation and usage are removed or deprecated consistently.