docs: update MR template to discourage closing companion issue

Description

This MR changes the MR template and suggests avoiding the use of Closes or Resolves in front of an issue link.

Here is a snippet from our contributing guidelines:

In this project, we don't close issues automatically when the MR gets merged. Instead, we close the issues when the MR change is released. Please replace Closes # in the MR description with Relates to #.

https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/blob/main/CONTRIBUTING.md?ref_type=heads#opening-merge-requests

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap