GitLab: default merge request template

Author: greenrobot

.gitlab/merge_request_templates/Default.md

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+## What does this MR do?
+
+<!-- Briefly describe what this MR is about. -->
+
+## Author's checklist
+
+- [ ] The MR 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/changed behavior; all test pass.
+  * My code conforms to our coding standards and guidelines.
+  * My changes are prepared in a way that makes the review straightforward for the reviewer.
+
+## 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
+  * Coverage percentages do not decrease
+  * 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)
+
+/assign me