Use git grep to search for conflict headers

What does this MR do?

• Fixes scripts/lint-conflicts.sh script to work in MacOS and also makes it faster and more reliable. Credit to @rspeicher
• Since new version is so much faster, search for conflict headers everywhere (with two exceptions)

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

Closes #24008 (closed)