#2500 Bump Requirement: pyvista >= 0.38.1

Before in the project meta data pyproject.toml under dependencies there is
the requirement `pyvista>=0.33.0`, see pyproject.toml, line 38.
But, this old version 0.33.0 lacks the function pyvista.plotting.colors.get_cycler
which ansys.mapdl.core.theme, line 16 wants to import. This functions was
just introduced in pyvista 0.38.1 with pull #3739, see definition of
get_cycler in the corresponding diff.

Bump pyvista >= 0.38.1 also for .ci\requirements_test_xvfb.txt
As this is still used in three locations in GitHub workflows.

.ci/requirements_test_xvfb.txt

@@ -1 +1 @@
-pyvista>=0.27.2
+pyvista>=0.38.1

pyproject.toml

@@ -35,7 +35,7 @@ dependencies = [
     "pyansys-tools-versioning>=0.3.3",
     "ansys-tools-path>=0.3.1",
     "pyiges[full]>=0.3.1", # Since v0.3.0, the 'full' flag is needed in order to install 'geomdl'
-    "pyvista>=0.33.0",
+    "pyvista>=0.38.1",
     "scipy>=1.3.0",  # for sparse (consider optional?)
     "tqdm>=4.45.0",
     "vtk>=9.0.0",