fix(1.x): textformatter `2.15` has breaking changes

[SychO9]

Changes proposed in this pull request: The recent 2.15 textformatter release seems to have some breaking changes:

• https://discuss.flarum.org/d/33746-an-error-occurred-while-trying-to-load-this-page/15
• https://github.com/s9e/TextFormatter/issues/226

Best to prevent updates to it for now until further notice for the 1.x line

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.