Prep for v1.39.0

[odow]

Basic

• version field of Project.toml has been updated
  - If a breaking change, increment the MAJOR field and reset others to 0
  - If adding new features, increment the MINOR field and reset PATCH to 0
  - If adding bug fixes or documentation changes, increment the PATCH field

Documentation

• Add a new entry to docs/src/changelog.md, following existing style

Tests

• https://github.com/jump-dev/MathOptInterface.jl/actions/runs/14371008998
• If new tests were added, ensure that MOI.Test.version_added is implemented.

[odow]

Merging because this is a small release with two things for @matbesancon, and @blegat's kwarg for Test.runtests.