Define minimum TF verison

[matobler]

With the integration of new tf.data functions we need to define the minimum version of TF that we will support. I also noticed that EfficientNet models trained on 2.5 will not work any longer on 2.3. So we probably want to set TF>2.5. We definitely want to remove TF 1.x support for MegaDetector (load MD model function).

[matobler]

I removed TF 1.x support in the loadMD function.

[tkswanson]

Added minimum version 2.5.0 to the description file

[tkswanson]

I just checked loadmodel, did you push it? It still says tfsession <- tf$compat$v1$Session()

[matobler]

The code is up to date. The .pb file used by MD is a TF 1.x format so we are using backwards compatibility implemented in TF 2.x to call 1.x functions (hence tf$compat$v1$...). This code will not work with TF 1.x. There used to be a check of which TF version was present and if 1.x was detected there was a different code to load the model, but since we depend on 2.x for other parts of the code (e.g. data generators) we might as well drop that. So I removed that part of the code.
We might need to re-write all of this for MD v5.