OpenSim updated to use Simbody#781

[fcanderson]

Brief summary of changes

This commit updates the dependency GIT_TAG for Simbody (see https://github.com/opensim-org/opensim-core/blob/main/dependencies/CMakeLists.txt#L186) to use https://github.com/simbody/simbody/commit/6e7913c3851105f3d24b3fad2fdc4a42a7d86f50.

https://github.com/simbody/simbody/commit/6e7913c3851105f3d24b3fad2fdc4a42a7d86f50 addresses inconsistent behavior in std::ostream::setprecision(int p) when p < 1 across operating systems (e.g., Windows 11 vs. Windows 10) and build types (e.g., Release vs Debug). See https://github.com/simbody/simbody/pull/781.

This update requires no changes to existing OpenSim code.

Testing I've completed

• I updated subunit test (https://github.com/simbody/simbody/blob/master/SimTKcommon/tests/TestXml.cpp#L537 ).
• simbody unit tests pass.
• opensim-core unit tests pass.

CHANGELOG.md (choose one)

• No need to update the change log because, with the exception of https://github.com/opensim-org/opensim-core/blob/main/dependencies/CMakeLists.txt#L186, no changes were made to OpenSim main.

---

This change is