GitLab: add merge request template

Author: greenrobot-team

.gitlab/merge_request_templates/Default.md

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+## What does this merge request do?
+
+Addresses objectbox/objectbox-swift#NUMBER+
+
+<!-- Briefly describe what this merge request is about. -->
+
+## Author's checklist
+
+- [ ] This merge request fully addresses the requirements of the associated task
+- [ ] I did a self-review of the changes and did not spot any issues. Among others, this includes:
+  * I added unit tests for new or changed behavior; all tests pass.
+  * My code conforms to our coding standards and guidelines.
+  * My changes are prepared (focused commits, good messages) so reviewing them is easy for the reviewer.
+- [ ] I assigned a reviewer to request review.
+
+## Review checklist
+
+- [ ] I reviewed all changes line-by-line and addressed relevant issues
+- [ ] The requirements of the associated task are fully met
+- [ ] I can confirm that:  
+  * CI passes
+  * New code conforms to standards and guidelines
+  * If applicable, additional checks were done for special code changes (e.g. core performance, binary size, OSS licenses)