DEP: Enforce deprecation of mangle_dup cols and convert_float in read_excel
I'll take care of the mangle dup cols within csv in a follow up when the other csv deprecations are merged

#41176 #34418

Thanks @phofl  Coming from pandas v1:

We would get `float(cell.value)` then `str(...)` it via `dtype=str` and get `1.34e+56` as output.
Now we get an `int` of `133999999999999996199207003536481209407184768050474778624` with no way of getting the real value anymore.

![image](https://github.com/pandas-dev/pandas/assets/53433222/2db3db0d-7bd5-44f9-9d0e-67262c5431db)

openpyxl `3.1.3` or `3.0.10` doesn't make a difference.