Prep for v1.21.0

[odow]

TODO

• Fix test_modification_objective_scalarquadraticcoefficientchange #2320
• [Utilities] add more distance_to_set implementations #2314
• [FileFormats] revert change to extract_function_and_set #2321
• [Test] fix isapprox in test_modification_xxx_scalarquadraticcoefficientchange #2322

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/6621005584
• https://github.com/jump-dev/MathOptInterface.jl/actions/runs/6622372171
• https://github.com/jump-dev/MathOptInterface.jl/actions/runs/6631821162
• https://github.com/jump-dev/MathOptInterface.jl/actions/runs/6633588530

[odow]

I think the PolyJuMP failure is a problem. It looks like it tried to call

MathOptInterface.jl/src/FileFormats/MOF/write.jl

Line 57 in 96c724f

function _extract_function_and_set(expr::Expr)

[blegat]

Yes, it seems it didn't start with an underscore before ? We could also just drop support for the old NL syntax from PolyJuMP but I don't want to make a breaking release

[odow]

I've reverted these changes for now because of jump-dev/SCS.jl#284

[odow]

See jump-dev/SCS.jl#284 (comment)

[odow]

Okay, I think this is now ready to go