Remove legacy compatibility code for PHP versions < 8.1

[ihor-sviziev]

Description (*)

This pull request removes code that was designed to be ran on PHP <8.1. Also, removed checks for PHP_VERSION_ID contact availability, since it's available in all PHP versions

Related Pull Requests

Fixed Issues (if relevant)

1. Fixes magento/magento2#<issue_number>

Manual testing scenarios (*)

1. ...
2. ...

Questions or comments

Contribution checklist (*)

• [ ] Pull request has a meaningful description of its purpose
• [ ] All commits are accompanied by meaningful commit messages
• [ ] All new or changed code is covered with unit/integration tests (if applicable)
• [ ] README.md files for modified modules are updated and included in the pull request if any README.md predefined sections require an update
• [ ] All automated tests passed successfully (all builds are green)

Resolved issues:

1. [x] resolves magento/magento2#39891: Remove legacy compatibility code for PHP versions < 8.1