Rename default branch

[mormj]

Following the move across the open source community as described and executed here: gnuradio/volk#461 for the volk database

This needs to be done at a convenient time so as to not disrupt any impending release - preferable right after our 3.10 release, or if that is deferred then the next set of maint branch releases

As a side note, the github default is main - but since we make heavy use of maint- this will make tab completion slightly more cumbersome. Perhaps a devel or default, though that would depart from the standard main

[willcode]

If we change it, devel would be more accurate.