Standard / 19115-3 / Fix indexing of maintenance date (#9080)

Element was dateOfNextUpdate in ISO19139 and is maintenanceDate in ISO19115-3 

See https://github.com/geonetwork/core-geonetwork/blob/main/schemas/iso19115-3.2018/src/main/plugin/iso19115-3.2018/convert/ISO19139/mapping/core.xsl#L681-L682

Author: fxprunayre

schemas/iso19115-3.2018/src/main/plugin/iso19115-3.2018/index-fields/index.xsl

@@ -737,7 +737,7 @@
         <xsl:for-each select="*:resourceMaintenance/*">
           <maintenance type="object">{
             "frequency": "<xsl:value-of select="*:maintenanceAndUpdateFrequency/*/@codeListValue"/>"
-            <xsl:for-each select="*:dateOfNextUpdate[*/text() != '']">
+            <xsl:for-each select="*:maintenanceDate/*/cit:date[*/text() != '']">
               <xsl:variable name="dateOfNextUpdateZulu"
                             select="date-util:convertToISOZuluDateTime(*/text())"/>
               <xsl:if test="$dateOfNextUpdateZulu != ''">