Revert "5.4.0 (#270)"

[mcmire]

This reverts commit 9c9aff3. The subject of this commit indicates that 5.4.0 is being released, when it was actually 5.3.1.

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Note

Low Risk
Low risk: documentation/metadata-only changes, but could affect publishing if this package.json version is used for release automation.

Overview
Reverts an erroneous release bump by removing the 5.3.1 section from CHANGELOG.md and updating the [Unreleased] compare link to start from v5.3.0.

Resets package.json version from 5.3.1 back to 5.3.0, aligning package metadata with the corrected changelog state.

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