Follow up for Add CI script to detect unmapped rules

What does this MR do?

Do not run the mapping check on the main branch. This is a follow-up for !597 (merged).

What are the relevant issue numbers?

gitlab-org/gitlab#461799 (closed)

Does this MR meet the acceptance criteria?

• The test cases cover both positive and negative cases and are also annotated with appropriate semgrep annotations:
  • For positive cases: // ruleid: ...
  • For negative cases: // ok: ....
• Following metadata fields exist for the rule(s) added/updated in this MR:
  • owasp with both 2017 and 2021 mappings.
  • category: "security"
  • cwe
  • shortDescription
  • security-severity
• The message field is valid and contains a secure code example.
• Applicable license is mentioned in the rule if embedded/taken from external source.
• Relevant labels including workflow labels are appropriately selected.