Fix Issue 20874 - std.json.assign requires '@safe' and 'pure'

[MoonlightSentinel]

Currently targeting master because stable seems to be broken.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20874	normal	std.json.assign requires '@safe' and 'pure'

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#7503"