Update Go from 1.11 to 1.13

What does this MR do?

Updates Go from 1.11.5 to 1.13.6 as Go 1.11 already reached end of support.

What are the relevant issue numbers?

&2 (closed)

Does this MR meet the acceptance criteria?

• [n/a] Changelog entry added
• [n/a] Documentation created/updated for GitLab EE, if necessary
• [n/a] Documentation created/updated for this project, if necessary
• [n/a] Documentation reviewed by technical writer or follow-up review issue created
• [n/a] Tests added for this feature/bug
• Job definition updated, if necessary
  • [n/a] Job definition example
  • [n/a] CI Templates (to be removed)
  • [n/a] Vendored CI Templates (also in CE)
  • [n/a] Ensure that all of the ENV vars supported are propagated to Docker containers upon docker run in job definitions
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• [n/a] Security reports checked/validated by reviewer