Bug#975115: [Pkg-mozext-maintainers] Bug#975115: webext-dav4tbsync and webext-tbsync out of sync

[Carl Suster]

I've just noticed this message in the debug log for tbsync about the 
provider failing to load:

   ** Mon Nov 23 2020 17:43:57 GMT+1100 (AEDT) **
   [Initializing module] : <messenger>
   ** Mon Nov 23 2020 17:43:57 GMT+1100 (AEDT) **
   [TbSync init] : Done

   ** Mon Nov 23 2020 17:43:57 GMT+1100 (AEDT) **
   [EventLog] : FAILED to load provider <dav>
   API version mismatch, TbSync@Beta 2.4 vs dav@2.4