Make the color of zero-contribution-squares in the activity heatmap more subtle (#24758)

The previous color had a too high contrast with the background.

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Co-authored-by: silverwind <me@silverwind.io>

web_src/js/components/ActivityHeatmap.vue

@@ -31,8 +31,8 @@ export default {
   },
   data: () => ({
     colorRange: [
-      'var(--color-secondary-alpha-70)',
-      'var(--color-secondary-alpha-70)',
+      'var(--color-secondary-alpha-60)',
+      'var(--color-secondary-alpha-60)',
       'var(--color-primary-light-4)',
       'var(--color-primary-light-2)',
       'var(--color-primary)',