[docs-scanner] Time-relative version language in cache backends documentation

[docker-agent]

File: content/manuals/build/cache/backends/_index.md

Issue

The document contains time-relative language ("Since BuildKit v0.21") which violates STYLE.md's prohibition on such framing. This type of language becomes stale and confusing to readers who don't know what the "current" version is.

Since BuildKit v0.21, image-manifest is enabled by default.

Suggested fix

Remove the time-relative phrase and state the current default directly:

"The image-manifest parameter is enabled by default."

If version-specific behavior is important for users on older versions, add a note:

"The image-manifest parameter is enabled by default. (For BuildKit versions before v0.21, you must set this parameter explicitly.)"

This preserves the version information without framing it as a recent change.

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Found by nightly documentation freshness scanner