Rename blocking MRs to cross-project MR dependencies

What does this MR do?

Renames "Blocking merge requests" to "Cross-project Merge Request Dependencies"

In the new / edit page:

In the MR widget:

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

Closes #13035 (closed)