Replace infinite scroll with Load more button in logs list

What does this MR do and why?

Replaces GlInfiniteScroll with a Load more button in LogsList, following the Pajamas Design System guidance for "Loading more". Infinite scroll is deprecated for accessibility reasons (see linked epic), and the migration path was agreed in the issue discussion.

This also fixes a bug where nextPageToken was never cleared once the API stopped returning a token, causing the UI to keep refetching the same final page indefinitely instead of correctly hiding the load action.

Changes

• Replace GlInfiniteScroll with a GlButton (size="small" category="tertiary" variant="confirm"), shown only while there are more pages to load, mirroring the same pattern recently used for the What's New drawer.
• Rename bottomReached → loadMore.
• Fix nextPageToken handling so it's cleared (?? null) when the API no longer returns one.
• Remove now-unused listHeight height-tracking (was only needed to constrain GlInfiniteScroll's scroll container).
• Update logs_list_spec.js accordingly.

References

• #592978
• gitlab-org#6830

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Enable the observability_features feature flag (http://127.0.0.1:3000/rails/features/)
2. Navigate to Project > Monitor > Logs
3. Confirm a Load more button appears at the bottom of the list instead of infinite scroll, and that it disappears once all pages have loaded

Screencast