WFF holds back Wagtail 8

Just wanted to flag that the `wagtail>=6,<8` requirement on this package is preventing projects from upgrading to Wagtail v8.

```
Command failed: uv lock --upgrade-package wagtail
Using CPython 3.13.15 interpreter at: /opt/containerbase/tools/python/3.13.15/bin/python3
× No solution found when resolving dependencies:
╰─▶ Because wagtail-flexible-forms>=2.1.0 depends on wagtail>=6,<8 and your
project depends on wagtail==8.0, we can conclude that your project and
wagtail-flexible-forms>=2.1.0 are incompatible.
And because your project depends on wagtail-flexible-forms==2.1.0, we
can conclude that your project's requirements are unsatisfiable.
```

The fix might be as simple as removing the upper bound.

Start by locating the package metadata that declares wagtail-flexible-forms' `wagtail>=6,<8` requirement. Reproduce the reported `uv lock --upgrade-package wagtail` failure with Wagtail 8, then verify that the dependency resolution succeeds and the package remains compatible with its supported Wagtail versions.

Written by the indexing model from the issue text.