.github, nimble: bump Nim from 2.0.0 to 2.0.8

[ee7]

See the release posts:

• https://nim-lang.org/blog/2023/12/19/versions-1618-202-released.html
• https://nim-lang.org/blog/2024/04/16/versions-1620-204-released.html
• https://nim-lang.org/blog/2024/06/17/version-206-released.html
• https://nim-lang.org/blog/2024/07/03/version-208-released.html

and new commits.

This commit reflects the only change to the upstream std/json. The upstream std/parsejson wasn't changed.

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Previous Nim bump: https://github.com/exercism/configlet/pull/723