RFR: 8347620: Shenandoah: Use 'free' tag for free set related logging

[Y. Srinivas Ramakrishna]

On Mon, 13 Jan 2025 20:07:03 GMT, William Kemper <wkemper at openjdk.org> wrote:

> Without a distinguishing tag, debug logging is too voluminous to enable when we really only want the free set's debug messages.

Marked as reviewed by ysr (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/23086#pullrequestreview-2550863968