fix deprecated ref to tokenizer.max_len (#10220)

This is to fix deprecated reference to `tokenizer.max_len` with `tokenizer.model_max_length` - similar to [issue 8739](#8739) and [PR 8604](#8604). 
Example [here](https://colab.research.google.com/gist/poedator/f8776349e5c625ce287fc6fcd312fa1e/tokenizer-max_len-error-in-transformers_glue.ipynb). The error happens when `glue_convert_examples_to_features` is called without `max_length` parameter specified. In that case line 119 with wrong reference gets called. This simple fix should  do it.

src/transformers/data/processors/glue.py

@@ -116,7 +116,7 @@ def _glue_convert_examples_to_features(
     output_mode=None,
 ):
     if max_length is None:
-        max_length = tokenizer.max_len
+        max_length = tokenizer.model_max_length
 
     if task is not None:
         processor = glue_processors[task]()