prepare 4.8.1

CHANGELOG.txt

+4.8.1 (March 21, 2021): improved performance when transforming many OCC
+entities; fixed regression in high-order meshing of surfaces with singular
+parametrizations; small bug fixes.
+
 4.8.0 (March 2, 2021): new interactive and fully parametrizable definition of
 boundary conditions, materials, etc. through ONELAB variables; new API functions
 for creating trimmed BSpline/Bezier patches, perform raw triangulations and

doc/texinfo/version.texi

 @c This file was generated by cmake: do not edit manually!
 
-@set GMSH-VERSION 4.8.0 (development version)
+@set GMSH-VERSION 4.8.1 (development version)
 @set GITLAB-PREFIX https://gitlab.onelab.info/gmsh/gmsh/blob/master