Version 4: Set up workflow for creating v3 -> v4 pull requests

### Summary
Implement a workflow which automatically creates a PR for merging the v3 develop branch into the v4 develop branch each time a PR is merged into v3 develop. This will keep v4 up-to-date with v3 changes.

### 💬 Description
When we worked on v3, we implemented a workflow which created a PR for merging the v2 develop branch into v3 develop. This made sure that all changes that we added to v2 were also included in the v3 develop branch, to keep everything up-to-date. The equivalent will need doing for v3 -> v4.

Actions:
- Create a workflow which opens a PR for updating the v4 develop branch when a PR gets merged into the v3 develop branch
- I think [this might be the original workflow for v3](https://github.com/mi6/ic-ui-kit/pull/2703/files) - but I'm not sure if we changed it slightly later. Hopefully this will provide a good starting point for the v3 -> v4 workflow though

**Note: this will need to be done in both the ic-ui-kit and ic-design-system repos**

Review the workflow files changed in pull request #2703, then check how the workflow setup is organized in both the ic-ui-kit and ic-design-system repositories. Done means a merge into v3 develop automatically opens a pull request to update v4 develop in both repositories.

Written by the indexing model from the issue text.