Update template to v0.2.21

[scverse-bot]

cookiecutter-scverse released v0.2.21.

Changes

Dear scverse ecosystem developers,

we overhauled the automated template sync for our cookiecutter template.
Instead of a github action in your repo that runs a cronjob, PRs are now created in a centralized fashion by @scverse-bot.
We hope the new system finally works for all (most) repos and apologize for any "noise" PRs we may have created so far. Unfortunately, the system is a bit tricky to debug -- the only real way of testing it is to trigger an actual template sync.

In case you have any questions, feel free to reach out to @grst, for instance via the scverse zulip chat.

Additional remarks

• unsubscribe: If you don’t want to receive these PRs in the future,
  add skip: true to template-repos.yml using a PR or,
  if you never want to sync from the template again, delete your .cruft file.
• If there are merge conflicts,
  they either show up inline (>>>>>>>) or a .rej file will have been created for the respective files.
  You need to address these conflicts manually. Make sure to enable pre-commit.ci (see below) to detect such files.
• The scverse template works best when the pre-commit.ci, readthedocs and codecov services are enabled.
  Make sure to activate those apps if you haven't already.