chore: increase phpstan level to 6

[SychO9]

Changes proposed in this pull request:
Increases the level to 6 for better static code analysis. The plan was to keep going till level 8, but I noticed a lot of false positives on that level, and while most of them can be fixed by ignoring specific errors, I think it'd be better to stick to this level for now.

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)
• Related core extension PRs: (Remove if irrelevant)