Replace MD5 with SHA256 in generate_hash_id_from_traceback function

[cryptochecktool]

The current implementation of generate_hash_id_from_traceback uses MD5 for hashing. Please update the function to use SHA256 for improved security and collision resistance.
Code at:

opencensus-python/opencensus/trace/stack_trace.py

Line 184 in 2e0710b

m = hashlib.md5() # nosec