Upgrade gradle 5.1 to 5.6

What does this MR do?

Upgrades gradle from 5.3 to latest 5x patch version: 5.6. This fixes an issue where gradle can't handle non-ascii characters https://github.com/gradle/gradle/issues/3117#issuecomment-397616341

What are the relevant issue numbers?

gitlab-org/gitlab#32443 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• [-] Documentation created/updated for GitLab EE, if necessary
• [-] Documentation created/updated for this project, if necessary
• [-] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template (also in CE)
  • Job definition example
  • CI Templates (to be removed)
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer