Allow Developers to clear project runner caches by default

What does this MR do and why?

Developers often need to clear a project's runner cache when dependencies or tests leave it in a bad state. Previously, they had to ask a Maintainer to do so. This MR grants the existing clear_runner_cache permission to project Developers by default. Maintainers and Owners inherit it; Guests and Reporters remain denied.

The existing controller action, pipeline UI, and RunnerCacheClear GraphQL mutation now check this permission. Granular personal access tokens still require the clear_runner_cache scope. There is no project opt-in setting or API for configuring one.

Successful resets now record a project_runner_cache_cleared project audit event. Clearing a cache can also make the next pipeline, including a protected-branch pipeline, fetch dependencies again. The documentation recommends pinning dependencies to avoid fetching an unreviewed upstream release.

References

Resolves #221634

Screenshots or screen recordings

Not applicable. No new UI control was added; the existing Clear runner caches button becomes available to Developers.

How to set up and validate locally

1. As a Developer on a project, visit Build > Pipelines. Confirm Clear runner caches is visible and that clearing the cache succeeds without changing project settings.
2. Confirm a Maintainer or Owner can clear the cache, while a Guest or Reporter cannot.
3. Confirm the RunnerCacheClear GraphQL mutation follows the same role permission and that granular personal access tokens still require clear_runner_cache authorization.
4. On Premium or Ultimate, confirm a successful reset appears as project_runner_cache_cleared under the project's Secure > Audit events page.

MR acceptance checklist

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