chore: bump leptos from 0.6.12 to 0.6.15

Bumps [leptos](https://github.com/leptos-rs/leptos) from 0.6.12 to 0.6.15.
- [Release notes](https://github.com/leptos-rs/leptos/releases)
- [Commits](https://github.com/leptos-rs/leptos/commits)

---
updated-dependencies:
- dependency-name: leptos
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>