fix: gate json fetch timestamp option

[davide221]

What

Fix the Docker prebuild configure failure by only passing
DOWNLOAD_EXTRACT_TIMESTAMP to FetchContent_Declare() when the active CMake
version supports it.

The Docker images install Ubuntu 22.04's CMake 3.22.1, while
DOWNLOAD_EXTRACT_TIMESTAMP was added in CMake 3.24. CMake 3.22 interprets the
unknown keyword incorrectly and fails while configuring the json FetchContent
subbuild.

Validation

• git diff --check
• Ubuntu 22.04 / CMake 3.22.1 focused FetchContent configure: passed
• macOS / CMake 4.3.1 focused FetchContent configure: passed

[cubic-dev-ai]

No issues found across 1 file

_{Re-trigger cubic}