[#24789] Mention prism is the new default Go SDK runner in CHANGES.md

[lostluck]

Mention default runner change.
Light advice for migrating off of the direct runner.

Clarify scope of legacy runner deprecation mention to Python.

See #24789 for Prism replacement.

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[lostluck]

R: @damccorm

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[damccorm]

LGTM (though I'd appreciate the recommended change, it is optional)