Upgrade to Sonata v4/5

[RSalo]

Hi. There is composer exception:

  Problem 1
    - Root composer.json requires sonata-project/page-bundle ^3.25 -> satisfiable by sonata-project/page-bundle[3.25.0].
    - sonata-project/page-bundle 3.25.0 requires sonata-project/admin-bundle ^3.91.1 -> found sonata-project/admin-bundle[3.91.1, ..., 3.107.0] but it conflicts with your root composer.json require (^4.5).

Use the option --with-all-dependencies (-W) to allow upgrades, downgrades and removals for packages currently locked to specific versions.
You can also try re-running composer require with an explicit version constraint, e.g. "composer require sonata-project/page-bundle:*" to figure out if any version is installable, or "composer require sonata-project/page-bundle:^2.1" if you know which you need.

[VincentLanglet]

As you can see, SonataPageBundle does not support Sonata4, only Sonata3.
Some work have already been started to add the support on the next major version, but that's not a small task.
#1370

We're currently focusing on other bundles sonata-project/dev-kit#1715
but any help is welcomed.