[conf.py]: Update sphinx-rtd-theme Config

Replace the deprecated option
[`display_version`](https://sphinx-rtd-theme.readthedocs.io/en/3.0.0/configuring.html#confval-display_version)
by
[`version_selector`](https://sphinx-rtd-theme.readthedocs.io/en/3.0.0/configuring.html#confval-version_selector).

Author: andthum

docs/source/conf.py

@@ -285,7 +285,7 @@
 # the documentation of that theme.
 html_theme_options = {
     "logo_only": False,
-    "display_version": True,
+    "version_selector": True,
     "prev_next_buttons_location": "both",
     # Table-of-contents options.
     "collapse_navigation": True,