feat: re-enable tracking api.bitaps.com testnet explorer in watchdog canister

[maksymar]

This PR re-enables the api.bitaps.com explorer for the testnet.

It was previously disabled due to instability, but with the stricter height target calculation introduced in PR327, this should improve the stability of testnet alerts.

[github-actions]

canbench 🏋 (dir: .)

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: 561.73 M (-0.01%) (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: 87.01 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 (0.00%) (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 (0.02%) (change within noise threshold)
    heap_increase: 7 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: pre_upgrade_with_many_unstable_blocks
  total:
    instructions: 5.82 B (0.21%) (change within noise threshold)
    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)

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