[skip ci] Update CHANGES - Version 1.2.6.post1
[skip ci] Update CHANGES - Version 1.2.6.post1

Hotfix release of version 1.2.6, pinning niworkflows to a release version (instead
of the development branch, since #1459) and including to bugfixes.

* [PIN] NiWorkflows 0.5.2.post7 (`1bf4a21 <https://github.com/poldracklab/fmriprep/commit/1bf4a21cce62c4330510a9a8ae50db876fbc23b0>`__).
* [FIX] Bad ``fsnative`` replacement in CIfTI workflow (#1476) @oesteban
* [FIX] Avoid warning when generating boilerplate (#1464) @oesteban
poldracklab/fmriprep,poldracklab/fmriprep,oesteban/preprocessing-workflow,oesteban/preprocessing-workflow,poldracklab/preprocessing-workflow,oesteban/fmriprep,oesteban/fmriprep,poldracklab/preprocessing-workflow,oesteban/fmriprep,poldracklab/fmriprep