fix: Parse Deprecated sections in Google-style docstrings

The code to parse these sections already existed, but the "Deprecated"
section header was missing from the table of known section headers.
Being an unknown section, Griffe then treated this as an admonition.

Issue-352: #352

src/_griffe/docstrings/google.py

@@ -69,6 +69,7 @@
     "modules": DocstringSectionKind.modules,
     "warns": DocstringSectionKind.warns,
     "warnings": DocstringSectionKind.warns,
+    "deprecated": DocstringSectionKind.deprecated,
 }
 
 _BlockItem = tuple[int, list[str]]