Add changelog for 2021-05-29 Breaking Changes merge (qmk#12939)

* Add ChangeLog for 2021-05-29 Breaking Changes Merge: initial version

* Add recent develop changes

* Sort recent develop changes

* Remove sections for ChibiOS changes per tzarc

No ChibiOS changes this round.

* Add and sort recent develop changes

* add notes about keyboard moves/deletions

* import changelog for PR 12172

Documents the change to BOOTMAGIC_ENABLE.

* update section headings

* re-sort changelog

* add additional note regarding Bootmagic changes

* remove changelog timestamp

* update dates in main Breaking Changes docs

* fix broken section anchors in previous changelogs

* add link to backlight/eeprom patch to changelog

* highlight some more changes

* link PRs from section headers

Co-authored-by: Zach White <[email protected]>