[1.x] chore(core): bump app version

[DavideIadeluca]

Prepares https://github.com/flarum/framework/milestone/44

Changes proposed in this pull request:

Reviewers should focus on:

Screenshot

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)

[luceos]

We actually have a whole checklist to follow for releases. If you're going to contribute to releases, it would probably be time to look into getting you access to that. cc @imorland @SychO9 and @GreXXL .