[Frontiers in ...] Remove formatting for volume (#5580)

* Remove formatting for volume

It seems that the style for Frontiers ... has changed and that volume should no longer be in bold : https://www.frontiersin.org/about/author-guidelines

We're keeping journals in italics in line with published PDFs

frontiers-medical-journals.csl

@@ -132,7 +132,7 @@
             <date variable="issued" prefix=" (" suffix=")">
               <date-part name="year"/>
             </date>
-            <text variable="volume" font-weight="bold" suffix=":"/>
+            <text variable="volume" suffix=":"/>
           </group>
           <text variable="page" suffix="."/>
         </else>