Upgrade LabKit to v5.8.0

What does this MR do and why?

Upgrades gitlab-labkit from 5.6.1 to 5.8.0 and synchronizes Gemfile.lock and Gemfile.next.lock.

Carries two releases, both additive:

• 5.7.0: three new field constants from labkit-spec (GL_KEY_TYPE, GL_KEY_ID, SYSTEM). None removed or renamed.
• 5.8.0: Labkit::RateLimit::BlockSummary (gitlab-org/ruby/gems/labkit-ruby!399 (merged)). Nothing in gitlab-rails calls it until the follow-up MR for #638007.

Bundler cooldown

The Gemfile sets cooldown: 7, and 5.8.0 was published on 2026-11-04. The lockfiles were generated with bundle lock --conservative --update gitlab-labkit --cooldown 0. Without the bypass, the normal lock works from 2026-11-11.

The CI lockfile checks pass with 5.8.0 locked. The sha256 in both lockfiles matches the one RubyGems publishes for 5.8.0.

@tkuah approved the bypass for this release in !268007 (comment 4164587676).

Checklist

• bundle lock --print | diff Gemfile.lock - is empty
• BUNDLE_GEMFILE=Gemfile.next bundle lock --print --lockfile Gemfile.lock | diff Gemfile.next.lock - is empty
• Diff is 3 files: Gemfile, Gemfile.lock, Gemfile.next.lock
• Decision on the cooldown: bypass approved in !268007 (comment 4164587676)