RFE: change default branch to master

[mtbc]

It sits oddly to still be working from develop here when we don't much use dev_<n>_<n> and so many of the related repositories use master so maybe we could use master on this repository too and adjust CI accordingly.

[joshmoore]

No objection, but someone will have to take charge and follow through across all the devspaces, etc.

[mtbc]

Here and on ome/bioformats#3482 and ome/openmicroscopy#6195 given recent sociopolitical discussion perhaps trunk is a less objectionable alternative with the correct meaning.