Update to the new main default branch

[jtpio]

Looks like the default branch has been renamed to main, but some references still need to be updated.

[jtpio]

Looks like the CI failure is relevant and related to github-activity not able to find previous entries on the main branch. For example the output is empty when running locally:

$ github-activity jupyter-server/jupyter_releaser --auth <token> --since v0.3.0 --until v0.4.0 --branch main
Running search query:
repo:jupyter-server/jupyter_releaser


Found 11 items, which will take 1 pages
Found 11 items, which will take 1 pages

[blink1073]

Yeah, let's wait for a new release and then update that test to target PRs that are actually on the main branch. It would have been nice if the GitHub API would consider them to be the same branch.

[blink1073]

Thanks!