doc: rename master and devel branches

[edkerk]

Description of the issue:

• In line with https://github.com/github/renaming, the master branch can now be renamed to main, which is the new default branch name in GitHub. This will also be applied to standard-GEM.
• standard-GEM also specifies that the development branch should be named develop.

Expected feature/value/output:

main and develop branches

Current feature/value/output:

master and devel branches

To do:

• [x] Rename the branches on GitHub. See instructions.
• [x] Update functions and documentation references to the branch names.
• [x] As suggested, include instructions on how to update local branch names in the release notes of the next yeast-GEM version?

[edkerk]

Instructions how to change your local branches

To rename your local branches and connect them to the renamed main and develop branches, run the following code in your local git repository:

git branch -m master main
git branch -m devel develop
git fetch origin
git branch -u origin/main main
git branch -u origin/develop develop
git remote set-head origin -a