Add docs on MAVEN_CLI_OPTS to License Management CI example page

What does this MR do?

Add docs on new MAVEN_CLI_OPTS environment variable to License Management CI example page

What are the relevant issue numbers?

#6838 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• [ ] Tests added for this feature/bug
• [ ] Tested in all supported browsers
• Conforms to the code review guidelines
• [ ] Conforms to the merge request performance guidelines
• Conforms to the style guides
• [ ] Conforms to the database guides
• ~~[ ] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.~~
• EE specific content should be in the top level /ee folder
• [ ] For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer