clarify motivation for release 1.15.3

[kevinrue]

NEWS state:

• Revert environment for Linux to the one of Bioconductor release 3.18.

Clarify that this is due to the issue described in python-pillow/Pillow#8148

As such, we have not been able to identify a working conda environment for scvelo==0.3.2 on Linux, so we reverted to the latest known working environment.