ref-manual: IMAGE_TYPES, add tar.zst, delete elf

Update list of legal IMAGE_TYPES to match what's in
image_types.bbclass.

(From yocto-docs rev: 7706a240d6dbbace9a76b4b55427f91bd4afc6da)

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/ref-variables.xml

@@ -6638,7 +6638,6 @@
      cpio.lzma
      cpio.xz
      cramfs
-     elf
      ext2
      ext2.bz2
      ext2.gz
@@ -6662,6 +6661,7 @@
      tar.gz
      tar.lz4
      tar.xz
+     tar.zst
      ubi
      ubifs
      wic