Simplify release instructions

[maxkfranz]

Notes re. the content of the pull request.

• Set the default branch to unstable for feature releases.
• Set the default branch to master for patch releases.
• Simplify instructions: patch & feature only (no back-port)
• Delete backport issue template

Checklist

Author:

• The proper base branch has been selected. New features go on unstable. Bug-fix patches can go on either unstable or master.
• N/A -- Automated tests have been included in this pull request, if possible, for the new feature(s) or bug fix. Check this box if tests are not pragmatically possible (e.g. rendering features could include screenshots or videos instead of automated tests).
• N/A -- The associated GitHub issues are included (above).
• Notes have been included (above).

Reviewers:

• All automated checks are passing (green check next to latest commit).
• At least one reviewer has signed off on the pull request.
• For bug fixes: Just after this pull request is merged, it should be applied to both the master branch and the unstable branch. Normally, this just requires cherry-picking the corresponding merge commit from master to unstable -- or vice versa.

[maxkfranz]

Updated instructions:

Background

• Ensure that a milestone exists for the release you want to make, with all the issues for that release assigned in the milestone.
• Bug fixes should be applied to both the master and unstable branches. PRs can go on either branch, with the patch applied to the other branch after merging.
• When a patch release is made concurrently with a feature release, the patch release should be made first. Wait 5 minutes after the patch release completes before starting the feature release -- otherwise Zenodo doesn't pick up releases properly.

Patch version

1. Go to Actions > Feature release
2. Go to the 'Run workflow' dropdown
3. [Optional] The 'master' branch should be preselected for you
4. Press the green 'Run workflow' button
5. Close the milestone for the release

Feature version

1. Go to Actions > Feature release
2. Go to the 'Run workflow' dropdown
3. [Optional] The 'unstable' branch should be preselected for you
4. Press the green 'Run workflow' button
5. Close the milestone for the release
6. Make the release announcement on the blog