chore: replace express with fastify for mock-api

[danez]

Ref: #5696

[github-actions]

📊 Benchmark results

Comparing with cf2013a

Package size: 248 MB

⬇️ 0.00% decrease vs. cf2013a

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Legend

• T-30 (7aab3dd): 305 MB
• T-29 (b52927b): 305 MB
• T-28 (27c08b3): 305 MB
• T-27 (0642908): 305 MB
• T-26 (8861256): 305 MB
• T-25 (9bc1ff4): 305 MB
• T-24 (0ad5872): 305 MB
• T-23 (97bbc6c): 305 MB
• T-22 (b4b2915): 318 MB
• T-21 (43cd94e): 238 MB
• T-20 (be998a0): 238 MB
• T-19 (01570e9): 238 MB
• T-18 (1991750): 238 MB
• T-17 (0457388): 234 MB
• T-16 (29035d7): 234 MB
• T-15 (0e3c917): 234 MB
• T-14 (f6faaf2): 234 MB
• T-13 (3c240bc): 242 MB
• T-12 (208465d): 242 MB
• T-11 (274070b): 234 MB
• T-10 (2b22ca7): 234 MB
• T-9 (2fc1ece): 234 MB
• T-8 (7f3cae4): 245 MB
• T-7 (fe60d05): 245 MB
• T-6 (5bae485): 245 MB
• T-5 (e919afe): 234 MB
• T-4 (a96978f): 234 MB
• T-3 (9adffcc): 234 MB
• T-2 (411096a): 241 MB
• T-1 (cf2013a): 248 MB
• T (current commit): 248 MB