refactor(dependencies): require pandas>=2.0.0

[wpbonelli]

Last #1788 was discussed (May 2023) it was proposed to wait one more release cycle before adding pandas as a core requirement, especially since pandas v2 came out in early April. In view of recent effort to optimize flopy file IO, now may be a good time. Some work underway to support MF6 PRT could also be simplified if pandas were present

• Add pandas>=2.0.0 as a core dependency
• Replace optional imports with standard imports
• Update docstrings (remove "if available" language)
• Remove alternative logic for pandas < 1.3.0 from sfroutputfile.py
• Remove requires_pkg("pandas") from autotests
• Update autotests to always test pandas path (where applicable)
• Remove pandas entries from flopy_method_dependencies.md

If support should be maintained for older pandas I can remove the >=2 constraint and restore sfroutputfile.py

[codecov]

Codecov Report

Merging #1887 (ac77a9b) into develop (7931112) will increase coverage by 0.0%.
The diff coverage is 93.3%.

@@           Coverage Diff           @@
##           develop   #1887   +/-   ##
=======================================
  Coverage     72.2%   72.2%           
=======================================
  Files          257     257           
  Lines        56508   56502    -6     
=======================================
+ Hits         40841   40842    +1     
+ Misses       15667   15660    -7     

Files Changed	Coverage Δ
flopy/utils/utl_import.py	68.1% <ø> (ø)
flopy/utils/sfroutputfile.py	76.6% <75.0%> (+1.2%)	⬆️
flopy/export/metadata.py	23.3% <100.0%> (+0.8%)	⬆️
flopy/mf6/utils/binaryfile_utils.py	10.6% <100.0%> (+0.4%)	⬆️
flopy/mf6/utils/mfobservation.py	13.5% <100.0%> (+0.4%)	⬆️
flopy/modflow/mfsfr2.py	87.5% <100.0%> (+<0.1%)	⬆️
flopy/modpath/mp6sim.py	84.1% <100.0%> (ø)
flopy/utils/flopy_io.py	75.4% <100.0%> (ø)
flopy/utils/mflistfile.py	71.3% <100.0%> (ø)
flopy/utils/mtlistfile.py	89.9% <100.0%> (-0.1%)	⬇️
... and 3 more