patch 8.1.2023: no test for synIDattr() returning "strikethrough"

Problem:    No test for synIDattr() returning "strikethrough".
Solution:   Extend the synIDattr() test. (Jaskaran Singh, closes #4929)
diff --git a/src/version.c b/src/version.c
index e5070c6..31f0bf4 100644
--- a/src/version.c
+++ b/src/version.c
@@ -758,6 +758,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2023,
+/**/
     2022,
 /**/
     2021,