Catch up changelog for putative 7.0.0 release

[apetro]

Catches up CHANGELOG.md to document the changes pending release, including an important breaking change, positing that the next release will therefore be a "major" release versioned 7.0.0.

This makes the changelog more current, but probably is not sufficient documentation of just what the nature of the breaking change is and what upgraders should do about it when they upgrade.

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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.

[ChristianMurphy]

LGTM, once linter is happy

[apetro]

Apparently the new latest failure is because the scope portion of the style(CHANGELOG): indent child bullets with 4 not 2 spaces commit message is not lowercase but must be. I'll see about rewriting history enough to fix that.