release(1.2.8): update mf6 version

[jdhughes-usgs]

Also added latest version of gsflow (v 2.3.0) and mfusg_gsi (v2.1.1).

[codecov]

Codecov Report

Merging #131 (8d0cffc) into master (6a93306) will increase coverage by 6.006%.
The diff coverage is 89.855%.

@@              Coverage Diff              @@
##            master      #131       +/-   ##
=============================================
+ Coverage   73.845%   79.852%   +6.006%     
=============================================
  Files           20        20               
  Lines         3464      3514       +50     
=============================================
+ Hits          2558      2806      +248     
+ Misses         906       708      -198     

Impacted Files	Coverage Δ
pymake/cmds/build.py	0.000% <0.000%> (ø)
pymake/utils/_meson_build.py	92.213% <70.000%> (+63.879%)	⬆️
pymake/pymake_build_apps.py	82.113% <72.727%> (+1.582%)	⬆️
pymake/config.py	100.000% <100.000%> (ø)
pymake/utils/_dag.py	72.916% <100.000%> (-8.705%)	⬇️
pymake/utils/_usgs_src_update.py	94.835% <100.000%> (+13.270%)	⬆️
pymake/utils/download.py	67.605% <100.000%> (-4.024%)	⬇️

... and 6 files with indirect coverage changes