Create pipelines for merge requests **only** when source branch is updated

What does this MR do?

Currently, pipelines for merge requests are triggered when source or target branch is updated. However, we should create only when source branch is updated, because it runs unexpected pipelines.

Related: https://gitlab.com/gitlab-org/gitlab-ee/issues/7380

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Tests added for this feature/bug
• [-] Tested in all supported browsers
• [-] [End to end tests added if this MR has Requires e2e tests label]-(https://about.gitlab.com/handbook/engineering/quality/test-engineering/)

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