In configs/common/packages.yaml, update to met@12 and metplus@6; add to neptune-python-env

[climbfuji]

Summary

In configs/common/packages.yaml, update met to version 12.0.1 and metplus to version 6.0.0. Add both packages to neptune-python-env.

Testing

• CI (this only tests compiling, not running met or metplus)
• NRL tested this spack-stack on Nautilus and it worked exactly in the same way as a conda-installed met/metplus

Applications affected

NEPTUNE, UFS

Systems affected

None

Dependencies

• waiting on Add MET v12.0.0 and METplus v6.0.0 spack#509

Issue(s) addressed

Working towards #1440

Checklist

• This PR addresses one issue/problem/enhancement, or has a very good reason for not doing so.
• These changes have been tested on the affected systems and applications.
• All dependency PRs/issues have been resolved and this PR can be merged.