Updates for AmberTools 24

[mattwthompson]

Packages aren't up yet: conda-forge/ambertools-feedstock#141

There doesn't appear to be a new GAFF bundled compared to 23 (#281)

$ head -n1 amber24_src/dat/leap/parm/gaff*dat                                                              14:38:42
==> amber24_src/dat/leap/parm/gaff.dat <==
AMBER General Force Field for organic molecules (Version 1.81, May 2017)

==> amber24_src/dat/leap/parm/gaff2.dat <==
AMBER General Force Field for organic molecules (Version 2.2.20, March 2021)

==> amber24_src/dat/leap/parm/gaff211.dat <==
AMBER General Force Field for organic molecules (Version 2.11, May 2016)

Haven't looked at what other change(s) are relevant here

[epretti]

GAFF 2.2.20 has been added in #368. AmberTools 25 has just been released although there does not appear to be a new GAFF, and conda-forge packages are not yet available. Once they become available, if there are any changes needed to support AmberTools 25, we could create a new issue detailing that.