Don't ask release managers to put plain text passwords in plain text files

### Describe the bug

The release guide currently says that [release managers _have to_ put plain text credentials into plain text files](https://github.com/apache/polaris/blob/b501bfc266147dc80b4e1b3a87b98edbb55fc98c/site/content/release-guide.md?plain=1#L61-L66).

That's not a good practice. Everything that a release manager executes (Gradle builds, Maven builds, tests, browsers) can read that information and the credentials are leaked.

### To Reproduce

_No response_

### Actual Behavior

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### Expected Behavior

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### Additional context

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### System information

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Start with site/content/release-guide.md, especially the linked lines 61-66, and review how the release process currently handles credentials during Gradle and Maven builds. Update the guide so it no longer instructs release managers to store credentials in plain-text files, then verify the revised instructions are secure and clear.

Written by the indexing model from the issue text.