refactor: remove $_SESSION from methods and functions

[pjsde]

Functions and methods should not directly call the $_SESSION array but use the session() helper to obtain their contents from the Services::Session library.

This avoids the need to override code if someone needs to organize the $_SESSION array in another way.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[kenjis]

This is an improvement.
Thank you.

[kenjis]

@pjsde We do not include merge commits in PR branches.
Can you do git rebase to remove the merge commit?
See https://github.com/codeigniter4/CodeIgniter4/blob/develop/contributing/workflow.md#updating-your-branch

[pjsde]

@pjsde We do not include merge commits in PR branches. Can you do git rebase to remove the merge commit

@kenjis Sorry, I hadn't noticed

[kenjis]

@pjsde Thank you! Glad you are able to use git rebase.