Add default issue and merge request templates

Relates to https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/1153

.gitlab/issue_templates/default.md

+Before raising an issue to the GitLab issue tracker, please read through our guide for finding help to determine the best place to post:
+
+* https://about.gitlab.com/getting-help/
+
+If you are experiencing an issue when using GitLab.com, your first port of call should be the Community Forum. Your issue may have already been reported there by another user. Please check:
+
+* https://forum.gitlab.com/
+
+If you feel that your issue can be categorized as a reproducible bug or a feature proposal, please use one of the issue templates provided and include as much information as possible.
+
+Thank you for helping to make GitLab a better product.

.gitlab/merge_request_templates/default.md

+## What does this MR do and why?
+
+_Describe in detail what your merge request does and why._
+
+<!--
+Please keep this description updated with any discussion that takes place so
+that reviewers can understand your intent. Keeping the description updated is
+especially important if they didn't participate in the discussion.
+-->
+
+## Screenshots or screen recordings
+
+_These are strongly recommended to assist reviewers and reduce the time to merge your change._
+
+<!--
+Please include any relevant screenshots or screen recordings that will assist
+reviewers and future readers. If you need help visually verifying the change,
+please leave a comment and ping a GitLab reviewer, maintainer, or MR coach.
+-->
+
+## How to set up and validate locally
+
+_Numbered steps to set up and validate the change are strongly suggested._
+
+<!--
+Example below:
+
+1. Enable the invite modal
+   ```ruby
+   Feature.enable(:invite_members_group_modal)
+   ```
+1. In rails console enable the experiment fully
+   ```ruby
+   Feature.enable(:member_areas_of_focus)
+   ```
+1. Visit any group or project member pages such as `http://127.0.0.1:3000/groups/flightjs/-/group_members`
+1. Click the `invite members` button.
+-->
+
+## MR acceptance checklist
+
+This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
+
+* [ ] I have evaluated the [MR acceptance checklist](https://docs.gitlab.com/ee/development/code_review.html#acceptance-checklist) for this MR.