Introduce consolidated ENABLE_CONTROL_FLOW_V2 flag.

The new toggle replaces ENABLE_COND_V2, ENABLE_WHILE_V2, and
ENABLE_TENSOR_ARRAY_V2. This means that these can't be toggled
independently anymore, notably that v1 TensorArrays can only be run
with v1 loops, and v2 TensorArrays with v2 loops.

This also introduces a corresponding environment variable
TF_ENABLE_CONTROL_FLOW_V2. I kept the old env vars as well in case
people are using them. They all flip the new single toggle now.

In addition, this change removes some while_v2 code for dealing with
v1 TensorArrays, since this is no longer a supported configuration.

PiperOrigin-RevId: 224862245