chore: bump default Oxia image to 0.16.7

[mattisonchao]

Motivation

• Update the default Oxia image used by the platform charts to the requested 0.16.7 release.

Modifications

• Bumped images.oxia.tag from 0.16.6 to 0.16.7 in sn-platform.
• Bumped images.oxia.tag from 0.16.6 to 0.16.7 in sn-platform-slim.

Testing

• Parsed both chart values files with Ruby YAML loader and verified images.oxia.tag is 0.16.7.
• Verified no remaining 0.16.6 chart defaults with rg.
• Ran git diff --check.

[github-actions]

@mattisonchao:Thanks for your contribution. For this PR, do we need to update docs?
(The PR template contains info about doc, which helps others know more about the changes. Can you provide doc-related info in this and future PR descriptions? Thanks)