Bump ffigen from 14.0.1 to 15.0.0 in /experimental/pedometer (#2477)

Bumps [ffigen](https://github.com/dart-lang/native/tree/main/pkgs) from 14.0.1 to 15.0.0.
<details>
<summary>Commits</summary>
<ul>
<li>See full diff in <a href="https://github.com/dart-lang/native/commits/HEAD/pkgs">compare view</a></li>
</ul>
</details>
<br />

[![Dependabot compatibility score](https://dependabot-badges.githubapp.com/badges/compatibility_score?dependency-name=ffigen&package-manager=pub&previous-version=14.0.1&new-version=15.0.0)](https://docs.github.com/en/github/managing-security-vulnerabilities/about-dependabot-security-updates#about-compatibility-scores)

Dependabot will resolve any conflicts with this PR as long as you don't alter it yourself. You can also trigger a rebase manually by commenting `@dependabot rebase`.

---

<details>
<summary>Dependabot commands and options</summary>
<br />

You can trigger Dependabot actions by commenting on this PR:
- `@dependabot rebase` will rebase this PR
- `@dependabot recreate` will recreate this PR, overwriting any edits that have been made to it
- `@dependabot merge` will merge this PR after your CI passes on it
- `@dependabot squash and merge` will squash and merge this PR after your CI passes on it
- `@dependabot cancel merge` will cancel a previously requested merge and block automerging
- `@dependabot reopen` will reopen this PR if it is closed
- `@dependabot close` will close this PR and stop Dependabot recreating it. You can achieve the same result by closing it manually
- `@dependabot show <dependency name> ignore conditions` will show all of the ignore conditions of the specified dependency
- `@dependabot ignore this major version` will close this PR and stop Dependabot creating any more for this major version (unless you reopen the PR or upgrade to it yourself)
- `@dependabot ignore this minor version` will close this PR and stop Dependabot creating any more for this minor version (unless you reopen the PR or upgrade to it yourself)
- `@dependabot ignore this dependency` will close this PR and stop Dependabot creating any more for this dependency (unless you reopen the PR or upgrade to it yourself)

</details>

Author: dependabot[bot]

experimental/pedometer/pubspec.yaml

@@ -14,7 +14,7 @@ dependencies:
   ffi: ^2.1.2
 
 dev_dependencies:
-  ffigen: ^14.0.0
+  ffigen: ^15.0.0
   jnigen: ^0.12.1
   flutter_test:
     sdk: flutter