fix: align Go toolchain versions for tomnomnom__gron task images

[owwll]

Fixes #9
Both task and task_cleanroom images for tomnomnom__gron.88a6234
were shipping with different Go versions:
task_cleanroom → go1.21.0
task → go1.24.2
This means a submission could be compiled against one toolchain
and evaluated against another, making benchmark results unreliable.
Fix: pinned both Dockerfiles to golang:1.21.0-alpine to match the
version declared in the project's go.mod.
Verified locally both containers now report the same Go version.

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