Python: Rename IDType_ID_GP.name_plural to grease_pencils

This renames `IDType_ID_GP.name_plural` for Grease Pencil from
`grease_pencil_v3` to `grease_pencil`.

Part of #125058.

Pull Request: https://projects.blender.org/blender/blender/pulls/146903

scripts/modules/bl_i18n_utils/settings.py

@@ -429,7 +429,6 @@ WARN_MSGID_NOT_CAPITALIZED_ALLOWED = {
     "curves",
     "fonts",
     "grease_pencils",
-    "grease_pencils_v3",
     "hair_curves",
     "ipos",
     "lattices",

source/blender/blenkernel/intern/grease_pencil.cc

@@ -358,7 +358,7 @@ IDTypeInfo IDType_ID_GP = {
     /*main_listbase_index*/ INDEX_ID_GP,
     /*struct_size*/ sizeof(GreasePencil),
     /*name*/ "GreasePencil",
-    /*name_plural*/ N_("grease_pencils_v3"),
+    /*name_plural*/ N_("grease_pencils"),
     /*translation_context*/ BLT_I18NCONTEXT_ID_GPENCIL,
     /*flags*/ IDTYPE_FLAGS_APPEND_IS_REUSABLE,
     /*asset_type_info*/ nullptr,