Add PHP 8 Union Types Support for tests in the codebase

[karyna-t]

In PHP 8, new Union types feature was introduced.
(see details https://wiki.php.net/rfc/union_types_v2)

Related issues:
#34263
#34264

In the scope of this ticket, we need to add Union Types support for the places where we're using an information about parameter's class - for Tests:

• https://github.com/magento/magento2/blob/platform-health/dev/tests/static/testsuite/Magento/Test/Integrity/PublicCodeTest.php#L128
• https://github.com/magento/magento2/blob/platform-health/dev/tests/static/framework/Magento/TestFramework/Integrity/Library/Injectable.php#L40
• https://github.com/magento/magento2/blob/platform-health/dev/tests/static/framework/Magento/CodeMessDetector/Rule/Design/CookieAndSessionMisuse.php
  (better to re-check other possible places)

Expected result (*)

Magento should not break when union types are used. Tests should properly check all dependencies and so on.

Actual result (*)

Right now Magento can't handle union types introduced in php 8.

[m2-assistant]

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[m2-assistant]

Hi @engcom-Dash. Thank you for working on this issue.
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[engcom-Lima]

@magento give me 2.4-develop instance