Fix setting avatar on login

[julien-nc]

• bump min NC version to 26 to make sure Php >= 8.0 and we get str_starts_with
• bump app version to 6.0.0 because we dropped compatibility with NC 25
• add support for avatar URL and base64
  • URL: content is downloaded and avatar is set on the fly
  • base64: support strings which begin with data:image/png;base64, and data:image/jpeg;base64,
• mention avatar support and details in the README

@juliushaertl What do you think about supporting URLs, in terms of security?

closes #835

[juliusknorr]

What do you think about supporting URLs, in terms of security?

I'd say this is fine as long as it requires configuring the avatar mapping. In that case the source would need to be somewhat trusted by the admin, but I'd say this is something we can require the admin of the Nextcloud/IdP to consider during configuration.

[konradmoesch]

Is there anything still to be done that I can help with? For me, this PR looks complete.

[andrevmatos]

This would be a great improvement, also to work with Authentik's user avatar file upload which becomes base64 urls