release: cli 0.6.21 + sandbox 0.2.34

[JannikSt]

Release bump for everything merged since v0.6.20.

• bump prime CLI/SDK to 0.6.21
• bump prime-sandboxes SDK to 0.2.34
• require prime-sandboxes>=0.2.34 from prime

Included changes:

• ENG-4782: prime train models — show FFT models by default #807 — prime train models shows FFT models by default
• improvement(images): pace bulk updates concurrently #803 — paced concurrent bulk image updates
• Lower sandbox default memory to 1 GB #804 — sandbox default memory lowered to 1 GB

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Note

Low Risk
Version-only changes with no runtime logic modified in this PR.

Overview
Release version bumps for the prime CLI/SDK and prime-sandboxes package, with no functional code changes in this diff.

Bumps __version__ in prime_cli to 0.6.21 and in prime_sandboxes to 0.2.34, and updates the prime package dependency from prime-sandboxes>=0.2.33 to >=0.2.34 so installs align with the new sandbox SDK release.

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