sr-ec and sr-el are not conform to BCP 47. https://tools.ietf.org/html/bcp47
BCP 47 explicit mentions sr-Latn and sr-Cyrl as examples.
This change requires 6839b8599458d978b0596de57c8cb8f31db5a2f6
in MediaWiki core included in MediaWiki 1.39.
The bidirectional fallback between the old and the new language codes
ensures that the special page aliases and the magic words supports both
language codes. For magic words this can be checked at
api.php?action=query&meta=siteinfo&siprop=magicwords&format=json
Just the order of the entries is different after this change.
Bug: T117845
Depends-On: I666fbdea89ccf21aab6ca1849adf22813dec052e
Change-Id: I409f384a5629741d45801d733c9293b270bca62e
