mirage/mirage: PR #1507: bump min ocaml-solo5 to avoid fast memory usage error

@28fe57e

Stages:

• ❌ x86_64-debian-10-4.13 (4m48s)
• ❌ arm64-debian-10-4.13 (2m07s)

History:

• 08/04/2024 17:53 ✔️ @38f92c8 (6m29s)
• 08/04/2024 17:52 🟠 @1088e5f
• 08/04/2024 17:29 ✔️ @ec44ee5 (6m34s)
• 08/04/2024 16:46 ✔️ @43b93dd (7m47s)
• 08/03/2024 13:37 ❌ @28fe57e (4m48s)
• 08/03/2024 08:27 ✔️ @e4a0b49 (7m03s)