[4.6] feat: force PHP default 32 chars length at 4 bits to Session ID

[kenjis]

Description Supersedes #9135 See #9116 and https://github.com/codeigniter4/CodeIgniter4/issues/9116#issuecomment-2300918329

These INI settings have been deprecated in PHP 8.4. This PR enforces the behavior in PHP 9.

Checklist:

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

[kenjis]

Added docs.

[github-actions[bot]]

:wave: Hi, @kenjis!

We detected conflicts in your PR against the base branch :speak_no_evil:
You may want to sync :arrows_counterclockwise: your branch with upstream!

Ref: Syncing Your Branch

[kenjis]

Rebased to resolve conflicts.