Upgrade project to PHP 8.3, add PHPStan, upgrade almost every dependency (#36)

* start updating to PHP 8.3

* taking off!

Co-authored-by: ErickSkrauch <erickskrauch@yandex.ru>
Signed-off-by: Octol1ttle <l1ttleofficial@outlook.com>

* dropped this

Signed-off-by: Octol1ttle <l1ttleofficial@outlook.com>

* migrate to symfonymailer

Signed-off-by: Octol1ttle <l1ttleofficial@outlook.com>

* this is so stupid 😭

Signed-off-by: Octol1ttle <l1ttleofficial@outlook.com>

* ah, free, at last.

Signed-off-by: Octol1ttle <l1ttleofficial@outlook.com>

* oh, Gabriel.

Signed-off-by: Octol1ttle <l1ttleofficial@outlook.com>

* now dawns thy reckoning.

Signed-off-by: Octol1ttle <l1ttleofficial@outlook.com>

* and thy gore shall GLISTEN before the temples of man.

Signed-off-by: Octol1ttle <l1ttleofficial@outlook.com>

* creature of steel.

Signed-off-by: Octol1ttle <l1ttleofficial@outlook.com>

* my gratitude upon thee for my freedom.

Signed-off-by: Octol1ttle <l1ttleofficial@outlook.com>

* but the crimes thy kind has committed against humanity

Signed-off-by: Octol1ttle <l1ttleofficial@outlook.com>

* Upgrade PHP-CS-Fixer and do fix the codebase

* First review round (maybe I have broken something)

* are NOT forgotten.

Signed-off-by: Octol1ttle <l1ttleofficial@outlook.com>

* Enable parallel PHP-CS-Fixer runner

* PHPStan level 1

* PHPStan level 2

* PHPStan level 3

* PHPStan level 4

* PHPStan level 5

* Levels 6 and 7 takes too much effort. Generate a baseline and fix them eventually

* Resolve TODO's related to the php-mock

* Drastically reduce baseline size with the Rector

* More code modernization with help of the Rector

* Update GitLab CI

---------

Signed-off-by: Octol1ttle <l1ttleofficial@outlook.com>
Co-authored-by: ErickSkrauch <erickskrauch@yandex.ru>

api/validators/PasswordRequiredValidator.php

@@ -24,12 +24,12 @@ class PasswordRequiredValidator extends Validator {
      */
     public $user = 'user';
 
-    public function init() {
+    public function init(): void {
         parent::init();
         $this->user = Instance::ensure($this->user, User::class);
     }
 
-    protected function validateValue($value) {
+    protected function validateValue($value): ?array {
         if ($this->user->can(P::ESCAPE_IDENTITY_VERIFICATION)) {
             return null;
         }