patch 9.0.0264: CI still runs on Ubuntu 18.04

Problem:    CI still runs on Ubuntu 18.04.
Solution:   Run CI on Ubuntu 20.04. (closes #10582)
diff --git a/src/version.c b/src/version.c
index ce61bbc..c17f7f8 100644
--- a/src/version.c
+++ b/src/version.c
@@ -732,6 +732,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    264,
+/**/
     263,
 /**/
     262,