Update sphinx-rtd-theme requirement from <0.5 to <0.6

Updates the requirements on [sphinx-rtd-theme](https://github.com/rtfd/sphinx_rtd_theme) to permit the latest version.
- [Release notes](https://github.com/rtfd/sphinx_rtd_theme/releases)
- [Changelog](https://github.com/readthedocs/sphinx_rtd_theme/blob/master/docs/changelog.rst)
- [Commits](readthedocs/sphinx_rtd_theme@0.1.8...0.5.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

doc-source/requirements.txt

@@ -7,7 +7,7 @@ sphinx-notfound-page
 sphinx-prompt>=1.2.0
 sphinx-tabs>=1.1.13
 sphinx_autodoc_typehints>=1.11.0
-sphinx_rtd_theme<0.5
+sphinx_rtd_theme<0.6
 sphinxcontrib-httpdomain>=1.7.0
 sphinxemoji>=0.1.6
 toctree_plus>=0.0.1