docs(changelog): Add release notes 6.0.0 to present to changelog file

[ChristianMurphy]

Based off the excellent feedback from #666 😈

ℹ️ About Keep a Changelog
This takes a different approach to keeping a changelog.
Rather than a primarily automated process based off commit messages. 🤖
This takes the purely human approach of letting the release manager add a description of changes. 📝

While still offering some recommendations on how to structure the changelog.

🔗 from Keep a Changelog docs

Guiding Principles

• Changelogs are for humans, not machines.
• There should be an entry for every single version.
• The same types of changes should be grouped.
• Versions and sections should be linkable.
• The latest version comes first.
• The release date of each versions is displayed.
• Mention whether you follow Semantic Versioning.

Types of changes

• Added for new features.
• Changed for changes in existing functionality.
• Deprecated for soon-to-be removed features.
• Removed for now removed features.
• Fixed for any bug fixes.
• Security in case of vulnerabilities.

🚧 Code changes

Copies release notes from GitHub releases 6.0.0 to present into CHANGLOG.md.

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Contributor License Agreement adherence:

• This Contribution is under the terms of Individual Contributor License Agreements (and also Corporate Contributor License Agreements to the extent applicable) appearing in the Apereo CLA roster.