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/testdir/test_syn_attr.vim b/src/testdir/test_syn_attr.vim
index cc20678..a27a2cb 100644
--- a/src/testdir/test_syn_attr.vim
+++ b/src/testdir/test_syn_attr.vim
@@ -13,6 +13,8 @@
   call assert_equal('1', synIDattr(hlID("Mine"), "underline", 'term'))
   call assert_equal('1', synIDattr(hlID("Mine"), "standout", 'cterm'))
   call assert_equal('1', synIDattr("Mine"->hlID(), "undercurl", 'gui'))
+  hi Mine gui=strikethrough
+  call assert_equal('1', synIDattr(hlID("Mine"), "strikethrough", 'gui'))
   hi Mine term=NONE cterm=NONE gui=NONE
   call assert_equal('', synIDattr(hlID("Mine"), "bold", 'term'))
   call assert_equal('', synIDattr(hlID("Mine"), "italic", 'cterm'))
@@ -21,6 +23,7 @@
   call assert_equal('', synIDattr(hlID("Mine"), "underline", 'term'))
   call assert_equal('', synIDattr(hlID("Mine"), "standout", 'cterm'))
   call assert_equal('', synIDattr(hlID("Mine"), "undercurl", 'gui'))
+  call assert_equal('', synIDattr(hlID("Mine"), "strikethrough", 'gui'))
 
   if has('gui')
     let fontname = getfontname()
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,