Fix relative links in Slack message

What does this MR do?

Issue #14244 (closed)

Our Slack notifications integration posts a message on Slack whenever there is a certain event on GitLab (comment on an issue, MR created...). However, links are currently included in their original Markdown form which leaves links as relative links. This MR updates relative links to absolute links so they are clickable from Slack.

Screenshots

Before

After

Original comment on GitLab

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team