chore: reduce the max instructions of heartbeats from 4B to 2B

[ielashi]

In an effort to make the FR of the Bitcoin subnet more stable, we reduce the amount of instructions we aim to execute in a single heartbeat execution from 4B instructions to 2B instructions. The hypothesis is that this will lead to a higher and more stable FR overall.

[github-actions]

canbench 🏋

No significant performance changes 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: 561.77 M (-0.00%) (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 (-0.00%) (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 (-0.00%) (change within noise threshold)
    heap_increase: 2 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: insert_block_headers_multiple_times
  total:
    instructions: 13.90 B (-0.00%) (change within noise threshold)
    heap_increase: 7 pages (no change)
    stable_memory_increase: 0 pages (no change)

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