Alerting: Fix bug in screenshot service using incorrect limit (#83786)

This commit fixes a bug in the screenshot service where
[alerting].concurrent_render_limit was used instead of
[rendering].concurrent_render_request_limit, as in the
docs.

pkg/services/screenshot/screenshot.go

@@ -122,7 +122,7 @@ func (s *HeadlessScreenshotService) Take(ctx context.Context, opts ScreenshotOpt
 		Width:           opts.Width,
 		Height:          opts.Height,
 		Theme:           opts.Theme,
-		ConcurrentLimit: s.cfg.AlertingRenderLimit,
+		ConcurrentLimit: s.cfg.RendererConcurrentRequestLimit,
 		Path:            u.String(),
 	}
 

pkg/services/screenshot/screenshot_test.go

@@ -54,7 +54,7 @@ func TestHeadlessScreenshotService(t *testing.T) {
 		Height:          DefaultHeight,
 		Theme:           DefaultTheme,
 		Path:            "d-solo/foo/bar?from=now-6h&orgId=2&panelId=4&to=now-2h",
-		ConcurrentLimit: cfg.AlertingRenderLimit,
+		ConcurrentLimit: cfg.RendererConcurrentRequestLimit,
 	}
 
 	opts.From = "now-6h"