Use single branch instead of develop/master split.

[holtgrewe]

Is your feature request related to a problem? Please describe.
Separating develop and master is not so useful for our project. Also, the default Maven release cycle assumes a single branch only. Releasing requires going to the master branch and then

Describe the solution you'd like
Get rid of the develop branch and use master for development. Releases go to Maven central anyway. While we're at it, we can move away from legacy master to main.

Describe alternatives you've considered
Leave things as they are.

Additional context
N/A

CC @visze @pnrobinson

[pnrobinson]

I agree there is no big advantage in separating develop/master

[holtgrewe]

I'm removing the legacy master and develop branchs in favour of the now default main branch.