Create Citation.cff points to no longer default branch "MASTER"

Browser & OS: Chrome96: Windows 10

## Describe the bug
A clear and concise description of what the bug is.
New github repositories does not have the MASTER branch, rather MAIN.
The badge for the "Create CITATION.rff" on github repository settings still points to the MASTER-branch, giving a not-found condition
## Steps to Reproduce
1. Visit Settings for github repository without an attached Citation.cff-file ( my case: https://zenodo.org/account/settings/github/repository/bjartelund/PsychOrNot#)
2. Attempt to create one by pressing the "Create CITATION.cff" badge leads to not found page (404) on GitHub.

I tried skimming through the github page to make a patch, but could not find this code.

## Expected behavior
Should lead to the create new file page of github

## Additional context
The problem is in the hardcoded `master` in [this line](https://github.com/inveniosoftware/invenio-github/blob/master/invenio_github/templates/invenio_github/settings/view.html#L176).

Kudos to @bjartelund