A cleanup for MDEV-19897 Rename source code variable names from utf8 to utf8mb3

`cmake . -DWITH_EXTRA_CHARSETS=complex` did not compile.
Fixing forgotten "utf8" to "utf8mb3" in definitions of
CHARSET and CHARSETS_AVAILABLE in cmake/character_sets.cmake

cmake/character_sets.cmake

@@ -22,10 +22,10 @@ IF(NOT DEFAULT_COLLATION)
   SET(DEFAULT_COLLATION "latin1_swedish_ci")
 ENDIF()
 
-SET(CHARSETS ${DEFAULT_CHARSET} latin1 utf8 utf8mb4)
+SET(CHARSETS ${DEFAULT_CHARSET} latin1 utf8mb3 utf8mb4)
 SET(CHARSETS_COMPLEX
   big5 cp1250 cp932 eucjpms euckr gb2312 gbk latin1 latin2
-  sjis tis620 ucs2 ujis utf8 utf8mb4 utf16 utf16le utf32)
+  sjis tis620 ucs2 ujis utf8mb3 utf8mb4 utf16 utf16le utf32)
 
 SET(CHARSETS_AVAILABLE 
 binary armscii8 ascii big5 cp1250 cp1251 cp1256 cp1257