Update dependency dagger/dagger to v0.21.8

[renovate]

This PR contains the following updates:

Package	Update	Change
dagger/dagger	patch	0.21.7 → v0.21.8
dagger/dagger	patch	v0.21.7 → v0.21.8

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Release Notes

dagger/dagger (dagger/dagger)

v0.21.8

Compare Source

v0.21.8 - 2026-07-29

Changed

• Changeset diffs are now computed from filesystem metadata instead of full-content comparison, significantly speeding up diff computation for large directories. by @​marcosnils in
  #​13615

Fixed

• Dockerfile build layer caching so unrelated build-context changes no longer bust the cache: COPY-ed directories now get a content-based cache identity, re-keying downstream steps only when copied content
  actually changes. by @​marcosnils in #​13765
• The --x-release CLI re-exec so _EXPERIMENTAL_DAGGER_RUNNER_HOST is preserved (with a warning) instead of being stripped, and clarified its startup message to avoid implying it runs from any build. by
  @​tiborvass in #​13752
• Silent SDK sessions no longer retain frontend telemetry in memory while Cloud and OTLP export remain enabled. DAGGER_SILENT is now honored as the equivalent of --silent. by @​sipsma in
  #​13762

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