fix(floodsub): remove rand 0.7 dependency

[pengqima]

Description

Replace floodsub's cuckoo filter with a bounded cache of 64-bit message hashes.

The new cache preserves the previous approximate limit of 2^20 entries, refreshes the eviction order when a duplicate is observed, and evicts the least recently seen hash when full. This keeps duplicate suppression bounded without retaining complete messages.

Removing cuckoofilter also removes its transitive dependency on rand 0.7.3, which is flagged by RUSTSEC-2026-0097. There are no public API changes.

Fixes #6419.

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Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates