fix(next/image): use `100% 100%` for placeholder image background-size when object-fit `fill` is used instead of cover (#67482)
### What?
- Fixes #67478
### Why?
- When an objectFit property that cannot be used for backgroundSize is
applied, I experienced an issue where the placeholder's size behaved
unexpectedly.
### How?
- Modify it to use 'cover' when the objectFit property cannot be used
for backgroundSize
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Co-authored-by: Sam Ko <sam@vercel.com>
Co-authored-by: Steven <steven@ceriously.com>