Update CHANGES to announce LaTeX deprecation of internal \dimen's (#11107)

CHANGES

@@ -10,6 +10,16 @@ Incompatible changes
 Deprecated
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+* LaTeX: some internal TeX ``\dimen`` registers are since 5.1.0 either not
+  used (and assigning them some dimension value is with no effect), or used
+  via changed names.  They never were mentioned in the docs but do have usable
+  public names, hence this deprecation notice.  They are
+  ``\sphinxverbatimsep``, ``\sphinxverbatimborder``, ``\sphinxshadowsep``,
+  ``\sphinxshadowsize``, and ``\sphinxshadowrule``.  It would be complicated
+  to let their use trigger some warning during PDF builds, so this will be the
+  sole announcement.  They will get removed from LaTeX support files at 7.0.0.
+  (refs: #11105)
+
 Features added
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