Stop logging "Rendering" twice per template at debug level on Rails 7.2 to 8.1

[PetrHeinz]

At debug level, Rails 7.2, 8.0 and 8.1 apps log every template render twice as "Rendering …", next to our single "Rendered …" line. Rails 7.1 and edge log none, which is what the subscriber intends. Evidence: one request in this repo's spec app on Rails 8.1.4 at debug level logs Rendering spec/support/rails/templates/template.html twice, then Rendered spec/support/rails/templates/template.html (0.1ms).

Since Rails 7.1, ActionView::LogSubscriber.attach_to subscribes two ActionView::LogSubscriber::Start listeners (template and layout) next to the subscriber, and those log the "Rendering" lines. LogtailLogSubscriber.attach_to removes them after calling super, but it looks them up with listeners_for, which skips silenced listeners. Rails 7.2 made Start silenceable (logger.nil? || !logger.debug?), and ActionView::Base.logger is still nil when the integration runs at boot. So nothing is found, and both Rails' own pair and the pair our super call adds stay subscribed.

• Look the listeners up with all_listeners_for where ActiveSupport has it (7.1+), falling back to listeners_for.
• Collect them into a new array with flat_map instead of concat-ing onto the first lookup: all_listeners_for returns the notifier's cached array, so concat would write into its cache.
• Tests: after boot no Start listener is left for render_template.action_view and render_layout.action_view, and a request at debug level logs one "Rendered" line and no "Rendering" line.

Behaviour and compatibility:

• At debug level on Rails 7.2 to 8.1 the "Rendering …" lines are gone, as on Rails 7.1 and edge. On Rails 7.0 and older the subscriber already drops them through its no-op log_rendering_start.
• Nothing changes at info level and above, where Start logs nothing anyway, or on other Rails versions.

Targets the logtail-rails 0.2.15 patch release. No dependencies on the other gems.

Commits and CI:

• Expect real response durations in the request specs is unrelated test maintenance that every logtail-rails PR needs right now: logtail-rack 0.2.8 times responses with the monotonic clock, which Timecop doesn't freeze, so three specs expecting "in 0.0ms" fail on every CI leg (main's last run still resolved logtail-rack 0.2.7). Keep Rails requests and their logs working with json 3 on Rails 8.0 and older #62 carries the identical commit, so the two merge cleanly in either order.
• The tests commit is expected to be red on CI, on the Rails 7.2, 8.0 and 8.1 legs. The fix commit follows it.

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