fix #9512

pin undici to 5.20.0 and in renovate. it seems like undici reverted the fix to a memory leak a while ago:

• undici issue: unidict fetch leaked if using Request object as argument nodejs/undici#1823
• initial fix: fix(fetch): weaker refs nodejs/undici#1824
• reverted in: fix: remove unnecessary WeakRef nodejs/undici#2000

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