Fix choosing SPDX license during plugin init

The keys used to be lower case, but were changed to mixed case
in this commit:

068e268c63

The identfier are now used as inputted by the user.

maubot/cli/util/spdx.py

@@ -36,10 +36,10 @@ def load() -> None:
 def get(id: str) -> dict[str, str]:
     if not spdx_list:
         load()
-    return spdx_list[id.lower()]
+    return spdx_list[id]
 
 
 def valid(id: str) -> bool:
     if not spdx_list:
         load()
-    return id.lower() in spdx_list
+    return id in spdx_list