MINOR: Fix unchecked warning in CredentialCache

[ex172000]

The CredentialCache uses generic and unavoidably introduced unchecked conversion. To avoid this, we have to make it explicit. This cache is only used in Scram though.

Committer Checklist (excluded from commit message)

• [x] Verify design and implementation
• [x] Verify test coverage and CI build status
• [x] Verify documentation (including upgrade notes)

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