Revert "[CI] Tensorflow version support upgrade from 2.1.0 to 2.3.1"

[leandron]

Reverts #6706, as it breaks five TFLite tests. This is also reported in #6754.

On my machine I can see these fails when I have TF 2.3.1 installed:

tests/python/frontend/tflite/test_forward.py::test_forward_convolution
tests/python/frontend/tflite/test_forward.py::test_forward_transpose_conv
tests/python/frontend/tflite/test_forward.py::test_forward_quantize_dequantize
tests/python/frontend/tflite/test_forward.py::test_forward_padv2 - Run...
tests/python/frontend/tflite/test_forward.py::test_forward_relu - Valu...

Note: I used GitHub's "revert" functionality here, to revert the whole #6706, and I think we can just use the same to re-merge it, when sources are ready to cope with TF 2.3.x.

cc @tqchen @ANSHUMAN87 @u99127

[ANSHUMAN87]

@leandron: Please refer #6774 for the fix.

[leandron]

#6774 fixes the issues reported in #6754, making this PR unnecessary, so I'm closing it.