chore: bump up canbench to 0.1.8

[maksymar]

This PR upgrades canbench crate to v.0.1.8 and pocket-ic to 6.0.0.

Canbench tests for this PR showed a regression for pre_upgrade_with_many_unstable_blocks, but that is expected since in canbench v.0.1.5 there was a fix for correctly accounting the instructions when writing stable memory (release), which is exactly what is happening in bitcoin canister pre_upgrade.

[github-actions]

canbench 🏋 (dir: .)

Significant performance change detected! ⚠️

./canbench_results.yml is up to date ✅

~/work/bitcoin-canister/bitcoin-canister/scripts ~/work/bitcoin-canister/bitcoin-canister
~/work/bitcoin-canister/bitcoin-canister

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Benchmark: insert_300_blocks
  total:
    instructions: 563.25 M (0.26%) (change within noise threshold)
    heap_increase: 6 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: get_metrics
  total:
    instructions: 87.00 M (0.01%) (change within noise threshold)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: insert_block_headers
  total:
    instructions: 3.90 B (0.09%) (change within noise threshold)
    heap_increase: 2 pages (no change)
    stable_memory_increase: 0 pages (no change)

---------------------------------------------------

Benchmark: insert_block_headers_multiple_times
  total:
    instructions: 13.92 B (0.13%) (change within noise threshold)
    heap_increase: 7 pages (no change)
    stable_memory_increase: 0 pages (no change)

---------------------------------------------------

Benchmark: pre_upgrade_with_many_unstable_blocks
  total:
    instructions: 5.97 B (regressed by 2.70%)
    heap_increase: 4097 pages (no change)
    stable_memory_increase: 1792 pages (no change)

  serialize_blocktree (scope):
    instructions: 2.39 B (0.45%) (change within noise threshold)
    heap_increase: 2048 pages (no change)
    stable_memory_increase: 0 pages (no change)

  serialize_blocktree_flatten (scope):
    instructions: 201.60 K (no change)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

  serialize_blocktree_serialize_seq (scope):
    instructions: 2.39 B (0.45%) (change within noise threshold)
    heap_increase: 2048 pages (no change)
    stable_memory_increase: 0 pages (no change)

---------------------------------------------------