revert: restore timeline diagram, gantt chart was misleading

The gantt bars spanned between date ranges which misrepresented
the data. The timeline diagram correctly maps each date to its
maximum certificate lifespan.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -17,18 +17,14 @@ TLS certificate lifespans are shrinking fast. The CA/Browser Forum passed [Ballo
 certctl is a self-hosted platform that automates the entire certificate lifecycle — from issuance through renewal to deployment — with zero human intervention. It works with any certificate authority, deploys to any server, and keeps private keys on your infrastructure where they belong.
 
 ```mermaid
-gantt
+timeline
     title TLS Certificate Maximum Lifespan (CA/Browser Forum Ballot SC-081v3)
-    dateFormat YYYY
-    axisFormat %Y
-    todayMarker off
-    section Lifespan
-        5 years (1825 days)    :done, 2015, 2018
-        825 days               :done, 2018, 2020
-        398 days               :active, 2020, 2026
-        200 days               :crit, 2026, 2027
-        100 days               :crit, 2027, 2029
-        47 days                :crit, 2029, 2031
+    2015 : 5 years
+    2018 : 825 days
+    2020 : 398 days
+    March 2026 : 200 days
+    March 2027 : 100 days
+    March 2029 : 47 days
 ```
 
 ## Documentation