Support different PR templates for master and LTS branches

[OhmSpectator]

We want to make our pull request process clearer by showing different instructions depending on the type of PR:

• PRs into master should follow the full template.
• Backports into LTS branches don’t always need a full description, but they must clearly say where the fix came from.

This idea came from our discussion about improving our PR templates.

Right now, GitHub only uses the PR template from the default branch. There’s no built-in way to switch templates based on the target branch.

A possible workaround is to use special URLs with template query parameters when creating a PR. We could also show the right URL after a git push, for example using a git hook.

Docs to start with: https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/creating-a-pull-request-template-for-your-repository