(DOCSP-33104) EOL Ubuntu 18.04 (#201)

Author: zach-carr

source/compatibility.txt

@@ -41,7 +41,7 @@ systems:
      - 12, 15
      - ``x86-64``, ``ARM``
    * - Ubuntu
-     - 18.04, 20.04, 22.04
+     - 20.04, 22.04
      - ``x86-64``, ``ARM``
 
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