Document how to set proxy for Maven dep scan

What does this MR do?

Provides MAVEN_CLI_OPTS required to let Maven project dependency scan use a proxy.

Related issues

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
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• ~"frontend"
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These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

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If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
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• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.