patch 8.1.1723: heredoc assignment has no room for new features

Problem:    Heredoc assignment has no room for new features. (FUJIWARA Takuya)
Solution:   Require the marker does not start with a lower case character.
            (closes #4705)
diff --git a/src/version.c b/src/version.c
index 36a7779..5dc4d0c 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1723,
+/**/
     1722,
 /**/
     1721,