docs: improve gitflow model for supported versions

docs: improve gitflow model for supported versions

Currently, supported versions are hosted on `support/<version>`. This bramch is intended to receive all commits and pull requests for compatible patches in order to prepare a release. The release would be made on the same bramch.

The issue with this model is that unline latest versions, support versions are not released on a "safe" branch. In order to resolve this issue and harmonize the way versions are prepared and released, the current commit move `support/<version>` to `develop/<version>` and add `master/<version>`.

Changes:
* Update CONTRIBUTING documentation to describe the `develop/...`+`master/...` approach
* Update GitHub pull request template to advise `develop/...` instead