Update both BMI C++ header copies to reflect virtual destructor

[PhilMiller]

Addresses the model engine element of #831.

These PRs need to be merged in tandem:

• NOAA-OWP/ngen#832
• NOAA-OWP/SLoTH#7
• https://github.com/NOAA-OWP/SoilMoistureProfiles/pull/17
• NOAA-OWP/SoilFreezeThaw#25
• NOAA-OWP/LGAR-C#26

Changes

• Modify the declaration to virtual ~Bmi()
• Update the submodule copy of the reference bmi-cxx/bmi.hxx to match upstream

Testing

1. Local CI
2. CI of SloTH, SMP, SFT, and LGAR-C, modified to cross-reference each others' updates

Notes

• All of the supported modules are going to have to make the switch in a somewhat coordinated fashion. This PR should perhaps encompass updating the submodule versions of any such that are affected, to make an atomic step forward.

Checklist

• [x] PR has an informative and human-readable title
• [x] Changes are limited to a single goal (no scope creep)
• [x] Code can be automatically merged (no conflicts)
• [x] Code follows project standards (link if applicable)
• [x] Passes all existing automated tests
• [x] Any change in functionality is tested
• [x] New functions are documented (with a description, list of inputs, and expected output)
• [x] Placeholder code is flagged / future todos are captured in comments
• [x] Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
• [x] Reviewers requested with the Reviewers tool :arrow_right: