Make xcube compatible with pandas 2.x and Python 3.12

[pont-us]

Closes #933 .

This PR removes the "<2" version pin from pandas in the environment.yml file, which also has the side effect that new xcube environments get Python 3.12 rather than 3.11 by default (since pandas 1.x was not compatible with Python 3.21). This PR includes the necessary updates to the unit tests and codebase to make the test suite run in a pandas 2 / Python 3.12 environment.

Checklist:

• ~[ ] Add unit tests and/or doctests in docstrings~ n/a
• ~[ ] Add docstrings and API docs for any new/modified user-facing classes and functions~ n/a
• ~[ ] New/modified features documented in docs/source/*~ n/a
• [x] Changes documented in CHANGES.md
• [x] GitHub CI passes
• [x] AppVeyor CI passes
• [x] Test coverage remains or increases (target 100%)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.20%. Comparing base (ad7adad) to head (5d6d6dd).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #934 +/- ## ======================================= Coverage 93.19% 93.20% ======================================= Files 457 457 Lines 35782 35784 +2 ======================================= + Hits 33347 33351 +4 + Misses 2435 2433 -2 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.