Update protobuf

[j-woz]

To 3.20.3 for TensorFlow 2.15 .

[j-woz]

Cf. #12

[jmohdyusof]

I thought someone had a working version newer than this? Latest is 5.27.x, should we try to find something later that works so we don't keep having to update so often?

[j-woz]

I think it's best to be flexible here and just track the TensorFlow requirements.