Pandas may fail to convert hifitime Epochs into datetime

[ChristopherRabotin]

Describe the bug

With some versions of pandas 2.0, the pd.to_datetime function in the plotting scripts may fail to convert the UTC epoch because pandas does not recognize UTC as a valid timezone. The fix is simple, but can't be done manually unless rewriting the plotting scripts.

To Reproduce

Steps to reproduce the behavior:

1. Run the test_orbit_determination.py with the latest pandas
2. Any of the plotting functions that uses to_datetime will fail.

Expected behavior

I expect the convert to work without failing

Code to reproduce the issue

The test above.

Platform

Linux

Additional context

N/A

[ChristopherRabotin]

This would be fixed with nyx-space/hifitime#185.