conf: set default language to 'en'

Starting with sphinx 5 setting language to 'None' results
in an error.

As the default translation is done in english, set language
to 'en' to prepare for future sphinx versions.

This also unbreaks our CI builds, which pulls in sphinx 5.

```
+ python3 -m sphinx -Wab html . _build/html/
Running Sphinx v5.0.1

Warning, treated as error:
Invalid configuration value found: 'language = None'. Update your configuration to a valid langauge code. Falling back to 'en' (English).
```

Change-Id: I7aeef9132214a30799b44b98b3ec5b663b3d366f
Signed-off-by: Alexander Martinz <amartinz@shiftphones.com>

conf.py

@@ -65,7 +65,7 @@
 #
 # This is also used if you do content translation via gettext catalogs.
 # Usually you set "language" from the command line for these cases.
-language = None
+language = 'en'
 
 # List of patterns, relative to source directory, that match files and
 # directories to ignore when looking for source files.