Move to Flask 3

[J08nY]

Currently, the web at https://sec-certs.org and (the page branch) runs on Flask 2.2 which is now quite old. There were/are multiple reasons for this, related to the breaking changes introduced in Flask 2.3 and 3.0:

• Flask-Menu breaks.
• Flask-Breadcrumbs break.
• Some other packages were broken for a while (Flask-Login AFAIR, story here).
• Other things may break.

Now that the situation is a bit more stable, we should update. The Menu and Breadcrumbs extension situation is not fixed and those packages seem unmaintained. However, their use in the web is limited (only for the Breadcrumbs functionality). We could implement this functionality ourselves quite easily or just ditch it for the time being (it is not critical).