Add the default .gitlab and .github issue and merge/pull request templates

Problem/Motivation

Opening a work item or a merge request on this project starts from an empty description. Nothing prompts the contributor for the Problem/Motivation, the steps to reproduce, the proposed resolution or the remaining tasks, so issues and merge requests arrive in whatever shape each contributor improvises, and reviewers have to ask for the same missing context every time.

GitLab reads a template out of the repository itself, per branch. The project does not carry those files yet.

Steps to reproduce

1. Open a new work item on this project on git.drupalcode.org — the description is empty.
2. Open a new merge request — no Checkpoints checklist is offered.
3. Look in the repository root — there is no .gitlab/ directory.

Proposed resolution

Add the default templates to the repository, on every branch that has an active .x-dev release:

• .gitlab/issue_templates/default.md — Problem/Motivation, Steps to reproduce, Proposed resolution, Remaining tasks, API changes, Data model changes, Release notes snippet.
• .gitlab/merge_request_templates/default.md — the Checkpoints checklist.
• .github/ISSUE_TEMPLATE/default.md and .github/pull_request_template.md — the same two shapes for the GitHub mirror of this repository.

All four are Markdown, which is what GitLab and GitHub render. The HTML issue summary template stays where it belongs, in the project's own issue-settings field on drupal.org.

Remaining tasks

• ✅ File an issue
• ❌ Addition/Change/Update/Fix
• ❌ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
• ➖ Developer Documentation support
• ➖ User Guide Documentation support
• ➖ UX/UI designer responsibilities
• ➖ Accessibility and Readability
• ❌ Reviewed by a human
• ❌ Code review by maintainers
• ❌ Full testing and approval
• ❌ Credit contributors
• ❌ Review with the product owner
• ❌ Update Release Notes
• ✅ Release

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• N/A