fix: usage of aria-hidden=false might cause inconsistent behavior

[YUCLing]

Fixes #4014

Changes proposed in this pull request: Replace all usage of aria-hidden=false with removing the attribute instead.

Reviewers should focus on:

Screenshot

QA Check if aria-hidden=false is still used in DOM tree.

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

• [x] 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)

[YUCLing]

seems like it can end up being false too

I just confirmed it, using false boolean value seems to remove the whole attribute in Mithril.

[hollsk]

Thanks @YUCLing (and @SychO9)! 😄