Fix for issue #2876: deprecate the use of valList as a direct child of elementSpec

[martindholmes]

This is the best solution I can think of for deprecation here.

[sydb]

I am not sure this is sufficient. Wouldn’t it be good to have something like

    <sch:rule context="tei:elementSpec/tei:valList">
      <sch:report test="true()">
        Use of &lt;valList> as a direct child of &lt;elementSpec> is deprecated
        and will be invalid after 2027-08-31. To express a controlled vocabulary
        as the content model of an element, place the &lt;valList> inside a
        &lt;content> element.
      </sch:report>
    </sch:rule>

in addition to the proposed remarks?