fix: migrate snapshot publishing to Central Portal

[Zxilly]

Summary

• Point snapshot publishing at https://central.sonatype.com/repository/maven-snapshots/ following Sonatype's migration documentation.
• Keep this change limited to the snapshot endpoint.

Validation

• Verified the diff contains one URL replacement and no workflow or credential changes.
• git diff --check passed.
• Local Gradle validation was blocked before configuration because services.gradle.org was unreachable; the existing PR test workflow will provide CI coverage.

Before merging

• Confirm that dev.zxilly has snapshots enabled in the correct Central Portal account.
• Confirm CI uses Central Portal publishing credentials. Legacy OSSRH credentials must not be tested against the new endpoint.
• Snapshot publication has not been run or verified. Merging would trigger the existing master prerelease workflow, so this remains a draft until publishing prerequisites are confirmed.

References: https://central.sonatype.org/publish/publish-portal-snapshots/ and https://github.com/gradle-nexus/publish-plugin#publishing-to-maven-central-via-sonatype-central

[mergify]

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