Bump graphviz from 0.20.1 to 0.21 in /requirements

Bumps [graphviz](https://github.com/xflr6/graphviz) from 0.20.1 to 0.21.
- [Changelog](https://github.com/xflr6/graphviz/blob/master/CHANGES.rst)
- [Commits](xflr6/graphviz@0.20.1...0.21)

---
updated-dependencies:
- dependency-name: graphviz
  dependency-version: '0.21'
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

requirements/requirements_docs.txt

@@ -1,6 +1,6 @@
 ansys-sphinx-theme[autoapi]==1.4.4
 enum-tools[sphinx]==0.13.0
-graphviz==0.20.1
+graphviz==0.21
 imageio==2.37.0
 imageio-ffmpeg==0.6.0
 pypandoc==1.15

requirements/requirements_test.txt

@@ -1,6 +1,6 @@
 ansys-platform-instancemanagement==1.1.2
 coverage==7.8.2
-graphviz==0.20.1
+graphviz==0.21
 imageio==2.37.0
 imageio-ffmpeg==0.6.0
 pypandoc_binary==1.15