ci: Upgrade Rust toolchain to 1.84

[berestovskyy]

The upgrade is required by the dfinity/stable-structures#267

[github-actions]

canbench 🏋 (dir: examples/btreemap_vs_hashmap)

Significant performance change detected! ⚠️

examples/btreemap_vs_hashmap/canbench_results.yml is up to date ✅

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

Benchmark: insert_users
  total:
    instructions: 2.55 B (-0.65%) (change within noise threshold)
    heap_increase: 871 pages (no change)
    stable_memory_increase: 0 pages (no change)

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

Benchmark: remove_users
  total:
    instructions: 2.11 B (1.78%) (change within noise threshold)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

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

Benchmark: pre_upgrade_bench
  total:
    instructions: 807.87 M (regressed by 8.59%)
    heap_increase: 519 pages (no change)
    stable_memory_increase: 184 pages (no change)

  serialize_state (scope):
    instructions: 792.94 M (regressed by 8.76%)
    heap_increase: 519 pages (no change)
    stable_memory_increase: 0 pages (no change)

  writing_to_stable_memory (scope):
    instructions: 14.93 M (-0.00%) (change within noise threshold)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 184 pages (no change)

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

[github-actions]

canbench 🏋 (dir: examples/fibonacci)

Significant performance change detected! ⚠️

examples/fibonacci/canbench_results.yml is up to date ✅

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

Benchmark: fibonacci_20
  total:
    instructions: 2187 (-1.66%) (change within noise threshold)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

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

Benchmark: fibonacci_45
  total:
    instructions: 2925 (improved by 2.86%)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

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

[berestovskyy]

I was not aware this PR duplicates #65

Since this PR is now passing all checks, we've decided to move forward with it.