all: Fix deprecated extlinks syntax for Sphinx 6.0 (#797)

The old syntax no longer works on recent Sphinx versions - see
sphinx-doc/sphinx#11094

Fails with:

```
Exception occurred:

  File "/home/max/.local/lib/python3.10/site-packages/sphinx/ext/extlinks.py", line 103, in role

    title = caption % part

TypeError: not all arguments converted during string formatting
```

Not 100% sure if the newer syntax works on all old versions, but if this
PR builds fine this should be good to go? In any case, if we ever want
to upgrade the Sphinx version (we probably should at some point), this
is required.

Author: GermanCoding

conf.py

@@ -114,9 +114,9 @@
 
 # Configure external links.
 extlinks = {
-    'issue': ('https://github.com/syncthing/syncthing/issues/%s', 'issue #'),
-    'user': ('https://github.com/%s', '@'),
-    'commit': ('https://github.com/syncthing/syncthing/commit/%s', ''),
+    'issue': ('https://github.com/syncthing/syncthing/issues/%s', 'issue #%s'),
+    'user': ('https://github.com/%s', '@%s'),
+    'commit': ('https://github.com/syncthing/syncthing/commit/%s', None),
 }
 
 # -- Options for HTML output ----------------------------------------------