Fix merge request chat messages for adding and removing approvals

What does this MR do?

Fix merge request chat messages for adding and removing approvals

With GitLab 13.2 we have ported optional MR approvals to Core. If a user uses an integration like Slack or Mattermost, the chat message will not send the correct chat message. We can simply backport the methods to CE in order to fix this problem.

See #28138 (closed)

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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