Improved Commit Message Generation in AcceptGitLabMergeRequestStep file

[AniketNS]

The commit message generation is refactored for better readability and efficiency in AcceptGitLabMergeRequestStep.java file. Made some changes in getCommitMessage() method.

Testing done

### Submitter checklist
- [ ] Link to relevant pull requests, esp. upstream and downstream changes
- [x] Make sure you are opening from a **topic/feature/bugfix branch** (right side) and not your main branch!
- [x] Ensure that the pull request title represents the desired changelog entry
- [ ] Please describe what you did
- [ ] Link to relevant issues in GitHub or Jira
- [ ] Ensure you have provided tests - that demonstrates feature works or fixes the issue