Add filtersClass prop to GlDashboardLayout

What does this MR do?

Adds a filtersClass prop to GlDashboardLayout, bound to the <section> that wraps the #filters slot.

Closes #3660 (closed)

Why

Slot content renders inside the filters <section>, so consumers had no way to style the wrapper itself. Some styles only work on the wrapper, e.g. position: sticky for a sticky filter bar. The only workaround was selecting gitlab-ui's [data-testid='filters-container'] from application CSS, which couples app styling to a test hook that can be renamed at any time.

The concrete driver is gitlab-org/gitlab!256156 (merged) (sticky filter bar on explore analytics dashboards), which currently reaches in via that data-testid.

Approach

• New optional String prop, default '', bound with :class alongside the existing static classes. Existing classes and data-testid are unchanged, so this is non-breaking.
• Follows the existing *Class prop pattern in gitlab-ui: GlDashboardPanel (containerClass, bodyContentClass) in the same dashboards/ directory, plus GlCard, GlTabs, GlEmptyState and GlAccordionItem.
• Unit tests cover the custom class being applied and the default classes being kept.
• Changeset added (minor).

Out of scope

header-container and actions-container have the same gap, as noted in the issue. I left them out to keep this MR small, and they can get the same treatment if someone needs it.

Screenshots or screen recordings

N/A. No visual change by default; the prop is empty unless a consumer sets it.

Integrations

No migration required (additive, non-breaking). Once released, gitlab-org/gitlab can replace the [data-testid='filters-container'] selector from !256156 with filters-class. I'll open that follow-up after the release.

• GitLab: mr_url

• CustomersDot: mr_url

• Duo UI: mr_url

• Status Page: mr_url

• Docs: mr_url

• Switchboard: mr_url

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Related to #3660 (closed)