[DOCS] Update alert colors to be on brand

[tyler-hoffman]

The purpose here is to make the version banner (for both past and prerelease) on brand. I didn't see other warnings in the docs, but if they exist, they will be recolored as well.

• [ ] Description of PR changes above includes a link to an existing GitHub issue
• [ ] PR title is prefixed with one of: [BUGFIX], [FEATURE], [DOCS], [MAINTENANCE], [CONTRIB]
• [ ] Code is linted - run invoke lint (uses black + ruff)
• [ ] Appropriate tests and docs have been updated

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