master branch renamed to stable

[RasmusKjeldgaard]

Renaming master as stable

Going forward the branch from which Kirby is released is named stable.

Because of a new branching strategy we now also have a new base branch called main. This is the target of non-breaking changes to the source code. Going forward breaking changes will target a branch that reflects an upcoming release, e.g. v6.0.0.

The main branch (and release branches, when a release has been finalised) will be merged to stable, once they are ready for release. This means that stable will reflect what is running in the wild while ongoing development work happens on feature branches with either the main branch or a specfic release branch as targets.
As a rule of thumb, developers never need to target the stable branch unless an immediate hotfix needs to be released.

Updating local copies of the repository

While the renaming itself is taken care of in this PR, local clones will need some updating to correctly target the new branch. Based on this small guide from Github, the following should be done locally for our repo:

$ git branch -m master stable
$ git fetch origin
$ git branch -u origin/stable stable
$ git remote set-head origin -a

An then optionally prune references to old branch:

$ git remote prune origin